Interaction between JavaScript and CSS. CSS properties for controlling web typography. the contents of this header will not be displayed

Interoperability between JavaScript and CSS

Cascading Style Sheets (CSS) are the standard visual representation HTML documents. Cascading style sheets are designed for use by designers: they allow you to precisely define fonts, colors, margins, alignment, border options, and even the coordinates of elements in a document.

But they are also of interest to programmers writing in client-side JavaScript because they allow animation effects, such as document contents fading out from the right edge, for example, or lists collapsing and expanding, allowing the user to control how much content is displayed. information.

Managing Inline Styles

The easiest way to control CSS styles is to manipulate the style attribute of individual document elements. Like most HTML attributes, the style attribute corresponds to a property of the same name on the Element object and can be manipulated in JavaScript scripts. However, the style property has one distinctive feature: its value is not a string, but a CSSStyleDeclaration object. The properties of this object represent the CSS properties defined in the HTML style attribute.

For example, to display the contents of a text element e in a large, bold, blue font, you can do the following to write the desired values ​​to properties that correspond to the font-size, font-weight, and color style properties:

E.style.fontSize = "24px"; e.style.fontWeight = "bold"; e.style.color = "blue";

When working with the style properties of a CSSStyleDeclaration object, remember that all values ​​must be specified as strings. In a style sheet or style attribute you can write:

Position: absolute; font-family: sans-serif; background-color: #ffffff;

To do the same for the e element in JavaScript, you need to enclose all values ​​in quotes:

E.style.position = "absolute"; e.style.fontFamily = "sans-serif"; e.style.backgroundColor = "#ffffff";

Note that semicolons are not included in string values. These are semicolons used in JavaScript syntax. Semicolons used in tables CSS styles, not needed in string values, installed using JavaScript.

Also, remember that all positioning properties must include units of measure. That is:

E.style.left = 300; // Incorrect: this is a number, not a string e.style.left = "300"; // Incorrect: missing units e.style.left = "300px"; // Right

Units are required when setting style properties in JavaScript - the same as when setting style properties in style sheets.

Many CSS style properties, such as font-size, contain a hyphen in their names. JavaScript interprets a hyphen as a minus sign, so the following expression cannot be written:

E.style.font-size = "24px"; // Syntax error!

Thus, the names of the properties of the CSSStyleDeclaration object are slightly different from the names of the actual CSS properties. If a CSS property name contains hyphens, the CSSStyleDeclaration object's property name is formed by removing the hyphens and capitalizing the letter immediately following each one. In other words, the border-left-width CSS property is accessed through the borderLeftWidth property, and the font-family CSS property can be accessed through the fontFamily property.

Additionally, when a CSS property, such as float, has a name that matches a JavaScript reserved word, the name is prefixed with "css" to create a valid CSSStyleDeclaration object property name. That is, to read or change the value CSS float properties element, you should use the cssFloat property of the CSSStyleDeclaration object.

The style attribute of an HTML element is its inline style, and it overrides any styling rules in CSS table. Inline styles are generally convenient to use for setting style values, and this is the approach used in all the examples above. Scripts can read the properties of a CSSStyleDeclaration object representing inline styles, but they return meaningful values ​​only if they were previously set by a JavaScript script or if the HTML element has an inline style attribute that sets the desired properties.

For example, a document might include a style sheet that sets the left margin for all paragraphs to 30 pixels, but reading the leftMargin property of one of these elements will return an empty line unless that paragraph has a style attribute that overrides the value set by the style sheet.

Reading an element's inline style is particularly challenging when reading style properties that have units or shorthand notation properties: the script must include more than one simple implementation parsing strings with CSS styles to allow extraction and further use values. In general, the inline element style is convenient to use only for setting styles.

Sometimes it is easier to read or write a single line into an element's inline style than to access the CSSStyleDeclaration object. To do this, you can use the getAttribute() and setAttribute() methods of the Element object or the cssText property of the CSSStyleDeclaration object:

// Both statements below set the style attribute // of element e to the string s: e.setAttribute("style", s); e.style.cssText = s; // Both instructions below get a value style attribute// element e as a string: s = e.getAttribute("style"); s = e.style.cssText;

Creation animation effects using CSS

One of the most typical areas applying CSS is the reproduction of visual animation effects. You can implement them using the setTimeout() or setInterval() methods, using them to organize multiple calls to a function that changes the inline style of an element.

// Makes element e relatively positionable and moves it left and right. // The first argument can be the element object or the value of the id attribute of the // desired element. If you pass a function as the second argument, it will be called with e // as an argument when the animation finishes playing. The third argument determines // the offset value of the element e. Defaults to 5 pixels. // The fourth argument determines how long the effect should play. // By default the effect lasts 500ms. function shake(e, oncomplete, distance, time) ( // Process arguments if (typeof e === "string") e = document.getElementByld(e); if (!time) time = 500; if (!distance) distance = 5; var originalStyle = e.style.cssText; // Save the original style e.style.position = "relative"; // Make it relative. var start = (new Date()).getTime(); / Remember the start of the animation animate(); // Start the animation // This function checks the elapsed time and changes the coordinates of e. // If it is time to end the animation, it restores the initial state // of the element e. Otherwise, it changes the coordinates of e and schedules its next call. function animate() ( var now = (new Date()).getTime(); // Get the current time var elapsed = now-start; // How much time has passed since the beginning? var fraction = elapsed / time; // Fraction of required time? if (fraction

Both shake() and fadeOut() accept an optional function callback in the second argument. If this function is specified, it will be called when the animation effect has finished playing. The element to which the animation effect was applied will be passed to the callback function as an argument. Next HTML markup creates a button that, when clicked, plays a shake effect and then a dissolve effect:

Click me!

Notice how similar the shake() and fadeOut() functions are to each other. They both can serve as templates for implementing similar animation effects using CSS properties.

Computed Styles

The element's style property specifies the element's inline style. It has an advantage over all style sheets and can be successfully used to set CSS properties to change the visual appearance of an element. However, in general case there is no point in calling it when you need to find out the actual styles applied to an element. What is required in this case is called computed style.

An element's computed style is the set of property values ​​that the browser has obtained (or calculated) from the inline style and all the rules from all style sheets that apply to the element: it is the set of properties actually used when rendering the element. Like inline styles, computed styles are represented by a CSSStyleDeclaration object. However, unlike inline styles, computed styles are read-only. These styles cannot be changed, but the computed CSSStyleDeclaration object allows you to know exactly the values ​​of the style properties that the browser used when rendering the corresponding element.

You can get the computed style of an element using the getComputedStyle() method of the Window object. The first argument to this method is the element whose calculated style you want to return. The second argument is required and usually passes null or the empty string, but it can also pass a string with a CSS pseudo-element name such as "::before", "::after", ":first-line" or " :first-letter":

Var title = document.getElementById("section1title"); var titlestyles = window.getComputedStyle(element, null);

The return value of the getComputedStyle() method is a CSSStyleDeclaration object representing all styles applied to the specified element (or pseudo-element). There are many significant differences between the CSSStyleDeclaration objects that represent inline styles and computed styles:

Computed style properties are read-only.

Computed styles properties have absolute values: Relative units of measurement such as percentages and points are converted to absolute values. Any property that specifies size (such as margin width or font size) will have a value expressed in pixels. That is, its value will be a string with the suffix "px", so you will need to implement it parsing, but you won't have to worry about defining and converting units of measurement. Property values ​​that define color will be returned in the format "rgb(#,#,#)" or "rgba(#,#,#,#)".

Properties that are shorthand notation are not evaluated - only the fundamental properties on which they are based. For example, you should not try to get the value of the margin property, but instead access the properties marginLeft, marginTop, etc.

The cssText property of the computed style is not defined.

Working with computed styles can be quite tricky, and accessing them may not always return the information you expect. Consider the font-family property as an example: it accepts a comma-separated list of font family names for compatibility. When reading the fontFamily property of a computed style, you are waiting for the value of the most specific font-family style applied to the element. And in this case, a value such as “arial, helvetica, sans-serif” may be returned, which does not say anything about the typeface of the actual font used.

Managing Style Sheets

So far we've seen how to set and get CSS style property values ​​and individual element classes. However, there is also the possibility of manipulating the CSS style sheets themselves. This is usually not necessary, but it can sometimes be useful, and this section will briefly outline the possible techniques.

When working with style sheets themselves, you'll encounter two types of objects. The first type is Element objects, which represent elements and that contain or reference style sheets. These are normal document elements, and if you define an id attribute on them, you can select them using the document.getElementById() method.

The second type of object is CSSStyleSheet objects, which represent the style sheets themselves. The document.styleSheets property returns a read-only array-like object containing CSSStyleSheet objects representing the document's style sheets. If you define a title attribute on an or element that defines or references a style sheet, that object will be available as a property of the CSSStyleSheet object with the name specified in the title attribute.

The following sections describe what operations can be performed on these elements and stylesheet objects.

Turning style sheets on or off

The simplest technique for working with style sheets is also the most portable and reliable. Elements and CSSStyleSheet objects define a disabled property that can be read and written by JavaScript scripts. As its name suggests, if the disabled property is set to true, the stylesheet is disabled and will be ignored by the browser.

This is clearly demonstrated by the disableStylesheet() function below. If you give it a number, it will interpret it as an index into the document.styleSheets array. If you pass it a string, it will interpret it as CSS selector, passing it to the document.querySelectorAll() method and setting the disabled property of all received elements to true:

Function disableStylesheet(ss) ( if (typeof ss === "number") document.styleSheets.disabled = true; else ( var sheets = document.querySelectorAll(ss); for(var i = 0; i

Getting, inserting, and removing rules from style sheets

In addition to providing the ability to enable or disable style sheets, the CSSStyleSheet object also defines an API for retrieving, inserting, and removing style rules from style sheets. IE8 and earlier versions implement a slightly different API than the standard one implemented by other browsers.

In general, directly manipulating style sheets is rarely useful. Rather than adding new rules to style sheets, it is usually better to leave them static and work with the element's className property. At the same time, if it is necessary to provide the user with the opportunity full control web page style sheets, you may need to arrange for dynamic table manipulation.

CSSStyleSheet objects are stored in the document.styleSheets array. The CSSStyle Sheet object has a cssRules property that stores an array of style rules:

Var firstRule = document.styleSheets.cssRules;

In IE this property is called rules, not cssRules.

The elements of the cssRules or rules array are CSSRule objects. In the standard API, a CSSRule object can represent any type of CSS rule, including @rules such as @import and @page directives. However, in IE, the rules array can only contain the actual stylesheet rules.

The CSSRule object has two properties that can be used in a portable way. (In the standard API, non-style rules do not have these properties, so you may need to skip them when traversing the style sheet.) The selectorText property is a CSS selector for of this rule, and the style property is a reference to a writable CSSStyleDeclaration object that describes the styles associated with this selector. As a reminder, CSSStyleDeclaration is the same type that is used to represent inline and computed styles.

The CSSStyleDeclaration object can be used to read existing or create new styles in rules. Often, when traversing a style sheet, it is the text of the rule itself that is of interest, not the parsed representation of it. In this case, you can use the cssText property of the CSSStyleDeclaration object, which contains the rules in the text representation.

In addition to the ability to retrieve and modify existing style sheet rules, you have the ability to add rules to and remove rules from the style sheet. The standard application interface defines insertRule() and deleteRule() methods that allow you to add and remove rules:

Document.styleSheets.insertRule("H1 ( text-weight: bold; )", 0);

The IE browser does not support the insertRule() and deleteRule() methods, but it defines addRule() and removeRule() functions that are almost equivalent to them. The only major difference (besides the function names) is that addRule() expects to receive a selector and style in text form in two separate arguments.

Creating New Style Sheets

Finally, you can create completely new style sheets and add them to your document. In most browsers, this operation is performed using standard techniques implemented in the DOM: a new element is created and inserted into the document in the section, then the contents of the style sheet are added using the innerHTML property. However, in IE8 and later earlier versions a new CSSStyleSheet object must be created using a non-standard method document.createStyleSheet(), and add style sheet text using the cssText property.

The example below demonstrates the creation of new tables:

// Adds a style sheet to the document and fills it with the specified styles. // The styles argument can be a string or an object. If it is a string, // it is interpreted as stylesheet text. If it is an object, then each // property of it must define a style rule to be added to the table. // The property names are selectors, and their values ​​are the corresponding styles function addStylesheet(styles) ( // First you need to create a new style sheet var styleElt, styleSheet; if (document.createStyleSheet) ( //If the IE API is defined, use it styleSheet = document .createStyleSheet(); ) else ( var head = document.getElementsByTagName("head"); styleElt = document.createElement("style"); // New element head.appendChild(styleElt); // Insert into // Now the new table is at the end of the array styleSheet = document.styleSheets;

) // Insert styles into the table if (typeof styles === "string") ( // Argument contains the text definition of the style sheet if (styleElt) styleElt.innerHTML = styles; else styleSheet.cssText = styles; // IE API ) else ( // Argument object with rules for insertion var i = 0; for(selector in styles) ( if (styleSheet.insertRule) ( var rule = selector + " (" + styles + ")"; styleSheet.insertRule(rule, i++ ); ) else ( styleSheet.addRule(selector, styles, i++); ) ) ) )

Good day, dear subscribers! In html it is possible to make an object visible or invisible, and there is various options

, and they display the content differently.

So, let's begin.

Let's imagine a situation: we need to fit a fairly large piece of text into a block of 200 by 300 pixels (in principle, it can be anything).

If we set the block with the help of styles to a width of 200 and a height of 300 pixels, then the height of it will stretch as much as the content in the block. But we need strictly 300 pixels in height! In this case, we do the following:

For our block, we set the overflow property with the value scroll . The block becomes the size we need, all the content fits in it, and scroll bars appear.

The overflow property values ​​are given below:

overflow – control the size of an object if its contents cannot be shown entirely.

Values:

auto – determined by the browser.

visible – the size is stretched to such an extent that all content is visible.

hidden – anything that goes beyond the boundaries of the element is simply not displayed.

scroll – All content is displayed and scroll bars appear.

For our specific case, an example:

XHTML

#st1( overflow: scroll; width:200px; height:300px; ) Quite a large fragment of text

There is a property that allows you to control the visibility of the element's content.

overflow – control the size of an object if its contents cannot be shown entirely.

For our specific case, an example:

visibility - controls in CSS the visibility of an element's content.

the contents of this header will not be displayed

The next property that allows you to control the visibility of a block is display .

overflow – control the size of an object if its contents cannot be shown entirely.

display - determines how the element will be displayed

none - the element is not displayed

block - breaks the line before and after the element (i.e. the element cannot be on the same line as other elements)

inline - does not break the line

Let's say we have text, among this text there is a tag or it can be a link, and we need the contents of this tag or link to be displayed on a separate line, then for them we will set display property with the value block. And vice versa, if it is necessary, let’s say that the tag is not on a separate line, but in the same one as the text, then we set its display property with the value inline .

For our specific case, an example:

will be on the same line with the text displayed on a separate line

The none value deserves special attention. If we set the display property to any element with the value none, then this element will not be displayed. Moreover, unlike the visibility property with the hidden value, this element will not be displayed completely (the visibility property with the hidden value will not display the contents of the element, and the element itself will take its place on the page).

Abstract: Access to style sheets. Style sheet properties. Adding and removing rules. Changing element styles. Element class names.

Let's take a look (in this moment) theoretical example - let there be a website where a series of technical articles is presented. We want to highlight some of these articles with a cool animated carousel, but what about users who don't have JavaScript enabled for some reason? Recalling what we've learned about unobtrusive JavaScript, we want the Web site's functionality to work for those users as well, but we may want to design the site differently for those users so that they can use the site comfortably, even without a carousel.

If you want to delete this rule, you can call the stylesheet.deleteRule(index) function, where index will be the index of the rule that will be deleted.

In the example of demonstrating articles, you can create a rule that makes the display property equal to none for all articles about HTML and JavaScript - look at the carousel example (http://dev.opera.com/articles/view/dynamic-style-css-javascript/carousel .html) to see it in action.

Note: IE does not implement rules according to standards. It uses rules instead of the cssRules attribute. IE also uses removeRule instead of deleteRule and addRule( selector , rule, index) instead of insertRule .

Changing Element Styles

You should now understand how to edit style sheets associated with a page and create and modify CSS rules within them. What if you want to change a specific element in the DOM? Using the DOM API you can access specific page elements. Returning to the carousel example, you can see that the functions are defined in such a way that when you click on an article, that article is highlighted, while the main text of the article is displayed below.

Using the DOM, we access the style object, which describes the style of the document. This style object is defined as CSSStyleDeclaration; detailed explanation This can be found in the W3C documentation for the CSSStyleDeclaration interface (http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleDeclaration). The style object doesn't work quite the same way as some of the other properties defined on the HTML element. Unlike element.href or element.id which return strings, element.style returns an object. This means that it is not possible to set a style by assigning a string to element.style .

The style object has attributes that correspond to various specified CSS properties. For example, style.color returns the color specified on the element. Doing element.style.color = "red";

You can dynamically change the style. Below is a function that turns an element's color red when passed the element's id.

function colorElementRed(id) ( var el = document.getElementById(id); el.style.color = "red"; )

You can also use setAttribute(key, value) to style an element. For example, set the color of an element to red by calling element.setAttribute("style", "color: red"); , but be careful as this removes any changes made to the style object. When an element's style is specified this way, it is the same as if we specified it as a style attribute declaration html element

. This style will only be applied when the importance and specificity of the rule is greater than other rules applied to the element (specificity is explained in Chapter 28 on CSS Inheritance and Cascading).

Some of you may be wondering what happens when a property is set

And on the same day, scientists made gravity worry. Coincidence? Not otherwise. The pattern from the previous article covered 90% of cases of multi-stage, and its simplicity is quite obvious. But would you like to hear about a pattern that works ~100% of the time and is also ridiculously complicated? Then I invite you to go with me to the next paragraph...

Missing 10%

While the CSS is loading, the parser is blocking, which means rendering is blocked for all subsequent content. According to the demo page from the above-mentioned article, rendering is divided into the following stages.

Mobile phones Computers

This is great on mobile, where each CSS for a section blocks itself and all subsequent sections, but on desktop, the CSS for main and comments in the left column blocks about-me from being rendered in the right column, even if the CSS for about-me is loaded first. This is because the blocking order is determined by the order in source code, but for this design it would be nice to have the right column appear before the left.

We need to build a dependency tree where the rendering of each element is blocked before the rendering of other specific elements. Also, dependencies should be able to change when the width of the viewport changes. Sounds tempting, doesn't it?

This can be done using custom CSS properties...

Custom CSS Properties

But for our conversation today it is enough to know this...

Html ( background: var(--gloop, red); )

Here we set the page background to the value from the --gloop custom property, and if it doesn't have one, set it to red as a fallback. As a result, the background will be red. But if you add:

:root ( --gloop: green; )

...we set the --gloop custom property to green , so the page is now green. But if you add:

:root ( --gloop: initial; )

The initial value is treated in a special way here. It actually overrides --gloop so that the page background is now red again.

Building a Rendering Dependency Tree Using Custom CSS Properties

While finishing this title, I was already glad how smart I am.

The HTML [ "/main.css", "/comments.css", "/about-me.css", "/footer.css" ].map(url => ( const link = document.createElement("link" ); link.rel = "stylesheet"; link.href = url; document.head.appendChild(link));

So you can load /initial.css using or embed it, since it still blocks rendering. But we load all other stylesheets asynchronously.

initial.css main, .comments, .about-me, footer ( display: none; ) :root ( --main-blocker: none; --comments-blocker: none; --about-me-blocker: none; - -footer-blocker: none; ) /* Other initial styles... */

We hide the sections that we are not yet ready to display, and then for each section we create a custom “blocker” property.

main.css :root ( --main-blocker: initial; ) main ( display: var(--main-blocker, block); ) /* Other main content styles... */

The main content does not have any display dependencies. Once the CSS is loaded, its blocker is canceled (with initial) and rendered.

comments.css :root ( --comments-blocker: var(--main-blocker); ) .comments ( display: var(--comments-blocker, block); ) /* Other comment styles... */

Comments should not be displayed before the main content, so the comment blocker is associated with --main-blocker . Block.comments becomes visible as soon as this CSS is loaded and --main-blocker is canceled

about-me.css :root ( --about-me-blocker: var(--comments-blocker); ) .about-me ( display: var(--about-me-blocker, block); )

Similar to the code above, .about-me depends on its CSS and comments. But when the page is wider, it's rendered in two columns, so we no longer need .about-me to depend on comments for display:

@media (min-width: 600px) ( :root ( --about-me-blocker: initial; ) ) /* Other styles for about-me... */

Ready! When the viewport width is over 600px , .about-me is rendered immediately after its CSS is loaded.

footer.css :root ( --footer-blocker: var(--main-blocker, var(--about-me-blocker)); ) footer ( display: var(--footer-blocker, block); ) / * Other styles... */

The footer should appear after the main content and about-me sections appear. To do this, --footer-blocker takes its value from --main-blocker , but immediately after canceling --main-blocker --footer-blocker falls back to the value taken from --about-me-blocker .

Demo

Required Chrome Canary or Firefox.

In this demo, the CSS is loaded asynchronously, taking approximately 0-5 seconds to load each file. Despite this, the page will never appear out of order, and each section will appear as soon as possible depending on the browser width.

But... is this practical?

This method is much more complex, with minimal benefits and huge backwards compatibility problems. But it demonstrates the power of CSS custom properties that solutions like Sass compile-time variables cannot.

If you really want to do something similar today, then you can shoulder the bulk of the work asynchronous loading CSS using loadCSS , and adding classes to once certain styles are loaded (), although this will lead to a lot of specific problems solved through hacks.

I guess this article can be considered a "fun example", but we've barely begun to discover the power of CSS custom properties.

Thanks to Remy Sharp for the corrections. Will I ever have an article without spelling errors? Nidokgah.

You can set attributes of the various styles directly in the markup or use a style sheet. The property can be used to assign a CSS class to a menu style that controls some aspect of the control"s appearance. The following example shows how to specify a property for a number of properties that you can then reference in a style sheet.

...

Note that it is a best practice to either specify inline styles in the markup or use the property and specify styles using a style sheet. It is not recommended that you both specify inline styles and use a style sheet, because unexpected behavior could result. For a general discussion of using CSS with server controls, see ASP.NET Web Server Controls and CSS Styles.

Menu Behavior and Styles

To control the appearance of the dynamic portion of the menu, you can use the style properties with the word "Dynamic" in the name:

The following example creates a collection of four styles that apply to the first four levels of menu items and could be referenced in a style sheet by using the value.

The first style in the collection applies to the first-level menu items; the second style applies to the second-level menu items, and so on. Note that there is no inheritance between level styles so that styles applied to one level do not affect subsequent levels.

The following example creates a collection of three styles that apply to the first three levels of menu items and could be referenced in a style sheet.