Does CSS Block Parsing and Rendering? A Detailed Analysis

CSS loading, analysis and application in the optimization of webpage performance: a key theme

The loading, analysis and application of CSS are the key links of web performance optimization. Understanding this process is essential for optimizing page loading time and improving user experience. The following will explore the analysis and rendering of the CSS to block the DOM, and the mechanism behind it.

CSS load and DOM parsing

DOM parsing

DOM (Document Object Model) Analysis refers to the process of browser converting the received HTML byte flow into the DOM tree.

During the analysis of the HTML document, when the parser encounters non -blocking resources (such as asynchronous script), it will try to download the resources parallel and continue to parse the document at the same time.
• Does CSS block the DOM analysis?

CSS itself does not block DOM analysis. In other words, the browser will continue to analyze HTML and build a DOM tree.

However, CSS will block the DOM rendering and the execution of JavaScript. This means that although the DOM tree can be constructed, the browser will not perform rendering operations before analyzing the relevant CSS (that is, the CSSOM tree). This is to ensure that the page is displayed correctly on the screen to avoid re -arranging and re -painting.
• When the browser analyzes HTML to generate the DOM tree, it also downloads CSS files parallel and starts to build a CSSOM (CSS object model). The construction of DOM and CSSOM occurs at the same time, which means that the download and analysis of CSS will not block the construction of the DOM.

CSSOM tree and rendering

cssom tree

CSSOM (CSS object model) is a data structure that is parallel to the DOM and contains all CSS information on the page. The browser uses it to build a rendering tree.

When the browser encounters
• tags or
tags, it will stop rendering, prioritize loading and parsing CSS, and build a CSSOM tree.
• <link></link> <style> The construction of the rendering tree

The rendering tree is the result of the combination of the DOM tree and the CSSOM tree, which means that the browser will be rendered.

Before the CSSOM tree is completed, the rendering tree cannot be constructed, because the rendering tree requires the style information of all DOM elements.
• This process can be represented by a chart:

Why does CSS load block JavaScript?

1. Make sure accurate style calculations : If JavaScript tries to modify the DOM or calculate style when the CSS is not fully loaded and parsed, the style information retrieved by JavaScript may be inaccurate. To avoid this, the browser ensures that all relevant CSS are loaded and analyzed before the execution of JavaScript, so that the script retrieval of the final style of the DOM element.
Avoid re -arrangement and re -painting
2. : If JavaScript is allowed to execute when the CSSOM is incomplete, it may modify the DOM based on incomplete style information. Once the CSSOM is completed, the browser may need to reinstate and redraw the rendered elements to significantly reduce the rendering efficiency. The dependence on the analysis order
: During the HTML parsing, when the browser encounters the
3. tag, it will immediately start loading CSS. When it encounters tags (no or attributes), it will suspend DOM parsing to execute the script. If the CSS is not fully loaded, the script execution may depend on incomplete style information. Therefore, the browser is waiting for the script before cssom is ready. <link href="..." rel="stylesheet"></link> <script> async Writing CSS in JavaScript will block the DOM rendering? defer
Whether to write a CSS style in JavaScript will block the DOM rendering depending on the application method and time of the style. This involves the rendering process of the browser, especially the relationship between JavaScript, CSS and DOM. Let's discuss it in detail:

Directly modify the element style

When the CSS style is directly applied by modifying the

attributes of the DOM element in JavaScript (for example,

), such operations usually do not block the DOM analysis. However, they may block the rendering process, because the browser must re -calculate the style and may trigger the reinstatement and re -painting:

style Restaurant element.style.color = 'red';: When the size, structure, or certain attributes of the element occur, the browser needs to re -calculate their position and size.

Draw
• : When the visual appearance of the element changes (such as color or border) but does not affect its size or structure, this will indicate the browser to re -draw elements.
Dynamic insertion or
• label If JavaScript dynamically adds
or

tags to

, it will affect the rendering: <style>

• Rendering block : The browser must be suspended to parse the newly inserted CSS rules before they can continue to rendering. This may lead to severe rendering delay, especially when the CSS file is large or the network condition is not good.
Performance impact
• : Repeated operation, such as inserting tags in the cycle, may lead to duplicate rearracking and re -painting, which seriously affects the performance of the page. <style>
CSS in JavaScript is not properly obstructed or delayed DOM rendering, especially when dynamic generation and application style.

Conclusion

Although CSS loading does not block the DOM analysis, it will block the DOM rendering. CSS loading will also block the follow -up of the follow -up JavaScript.

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