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Detailed explanation: Will css loading cause blocking?

小云云Original: 2017-12-05 16:07:002092browse

JS execution will block the parsing and rendering of the DOM tree, so will css loading block the parsing and rendering of the DOM tree? So, next I will do a test on the parsing and rendering of the DOM tree by css loading.

In order to complete this test, let’s do some popular science first, how to use chrome to set the download speed

1. Open the chrome console (press F12), you can see the picture below , focus on the place where I drew the red circle

##2. Click on the place where I drew the red circle (No throttling), you will see the picture below, we choose the GPRS option

3. In this way, the upper limit of our resource download speed will be limited to 20kb/s. Okay, then let’s get to our main topic

Will css loading block the parsing and rendering of the DOM tree?

Speak in code:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>css阻塞</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style>
      h1 {
        color: red !important
      }
    </style>
    <script>
      function h () {
        console.log(document.querySelectorAll(&#39;h1&#39;))
      }
      setTimeout(h, 0)
    </script>
    <link href="https://cdn.bootcss.com/bootstrap/4.0.0-alpha.6/css/bootstrap.css" rel="stylesheet">
  </head>
  <body>
    <h1>这是红色的</h1>
  </body>
</html>

Hypothesis: css loading will block DOM tree parsing And rendering

Assumption result: Before bootstrap.css is loaded, the following content will not be parsed and rendered, so what we see at the beginning should be a white screen, and h1 will not be displayed. And the result of console.log should be an empty array at this time.

Actual result: As shown below

Will css block DOM tree parsing?

We can see from the above picture that when the css has not been loaded, h1 is not displayed, but at this time the console output is as follows

It can be known that the DOM tree has at least been parsed to h1 at this time, but the css has not been loaded yet, which means that the css will not block the parsing of the DOM tree.

Will css loading block DOM tree rendering?

From the above picture, we can also see that when the css has not been loaded, the page displays a white screen. It is not until the css is loaded that the red font is displayed, that is Said that although the following content was parsed, it was not rendered. Therefore, css loading will block DOM tree rendering.

Personal evaluation of this mechanism

In fact, I think this may also be an optimization mechanism of the browser. Because when you load css, you may modify the style of the DOM node below. If the css loading does not block the rendering of the DOM tree, then after the css is loaded, the DOM tree may have to be redrawn or reflowed, which causes some problems. There is no necessary loss. So I simply parse the structure of the DOM tree first, complete the work that can be done, and then wait for your css to be loaded, and then render the DOM tree according to the final style. This approach will indeed be better in terms of performance. .

Will css loading block js running?

From the above inference, we can conclude that css loading will not block DOM tree parsing, but it will block DOM tree rendering. So, will css loading block js execution?

Similarly, verify through code.

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>css阻塞</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <script>
      console.log(&#39;before css&#39;)
      var startDate = new Date()
    </script>
    <link href="https://cdn.bootcss.com/bootstrap/4.0.0-alpha.6/css/bootstrap.css" rel="stylesheet">
  </head>
  <body>
    <h1>这是红色的</h1>
    <script>
      var endDate = new Date()
      console.log(&#39;after css&#39;)
      console.log(&#39;经过了&#39; + (endDate -startDate) + &#39;ms&#39;)
    </script>
  </body>
</html>

Assumption: css loading will block the subsequent js execution

Expected result: The js code behind the link should not run until the css loading is completed

Actual result:

We can see from the above picture that the js code located before the css loading statement is executed first, but the code located after the css loading statement is not executed until the css loading is completed. . This also shows that css loading will block the execution of subsequent js statements. See the picture below for detailed results (css loading took 5600+ms):

##Conclusion

From the above, we can The following conclusions are drawn:

1.css loading will not block the parsing of the DOM tree

2.css loading will block the rendering of the DOM tree

3.css loading will block the execution of subsequent js statements

Therefore, in order to avoid letting users see a long white screen time, we should improve the css loading speed as much as possible. For example, we can use the following methods:

1. Use CDN (because CDN will select the nearest node with cached content to provide you with resources based on your network conditions, so it can reduce loading time)
2. Compress css (can Use many packaging tools, such as webpack, gulp, etc., you can also turn on gzip compression)
3. Use cache reasonably (setting cache-control, expires, and E-tag are good, but you should pay attention to one problem, After the file is updated, you need to avoid the impact of caching. One solution is to add a version number after the file name)
4. Reduce the number of http requests, merge multiple css files, or simply directly. Written as inline style (one disadvantage of inline style is that it cannot be cached)

The above content is a detailed explanation of the blocking caused by css loading. I hope it will be helpful to everyone.

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