Understand the rendering process of html pages in preparation for learning front-end performance optimization (continued)_HTML/Xhtml_Web page production

Last night I wrote an essay about the rendering process of the browser, but it was only explained through a small piece of code. It did not pass the browser test. It was not convincing enough and there were many imperfections. Today I I tested it in the browser and shared the test results with everyone. The testing process may be a bit messy, I hope everyone understands.

Test browsers: Chrome v24.0.1312.52 m, Firefox v18.0, Opera v12.12.

In the WebKit kernel, when a web page is displayed, there will be a parser (Parser) to parse the html document, then generate a rendering tree (Render Tree), and finally render the page. This is done in a thread, so the two are not done at the same time.

I divided the following two situations and tested them in different browsers respectively.

The style file is in the head, and the other two script files are at the beginning of the body and the other at the bottom of the body. The style file is at the beginning of the body, and the location of the script file is the same as above.

The results of the test are as follows. In Chrome, the location of the style file will affect the download time of the image, but there is no difference between the two situations in the other two browsers. The following is the detailed testing process.

Test 1: The style file is in the head, and the other two script files are at the beginning of the body and the other at the bottom of the body.

Tested code:

Copy code

The code is as follows:

Hi, there!

Hi, again!

1. Testing in Chrome

After I opened the page in the browser, I quickly took a screenshot of the webpage, as shown below (click to view the larger image, the same below):

As can be seen from the picture above, the test.htm document has been loaded, and nothing is displayed on the page. example.css is in the pending state, but the last.js at the bottom has been loaded. This shows that Chrome has preloaded, downloaded in advance, and placed it in the browser's cache. Although last.js has been loaded, it has not been executed yet because the style file in front of it will block the execution of the script.

Next, when example.css is loaded, Hi there! is displayed on the screen, and the screenshot of the browser is as follows :

It can be seen from the network request that example.css has been loaded and other.js is in the pending state. However, the three images under the script tag have been downloaded at this time. This is due to the browser's preloading function. To. However, because the browser's rendering is blocked by the other.js script, these three pictures and the "Hi again" on them will not be displayed. In addition, the code in last.js has not been executed yet.

Next, when other.js is loaded, the browser will build a rendering tree, display "Hi again", and display the image. Since last.js has been downloaded previously, last.js will be executed immediately. The entire rendering process is completed. As shown below:

It can be seen from this that Chrome will preload the script resources in the body (the style file has not been tested). The JS dynamically loaded by the JavaScript script will not affect the download of the image file, but will affect the rendering of the image below it.

2. Test results in Firefox

Quickly take a screenshot after opening the page in Firefox, as shown below:

It is obviously different from Chrome, "Hi there!" is already displayed on the page, but the background color is white, indicating that the style file has not been downloaded yet. However, it will not be displayed in Chrome until the style file is loaded.

Next, when the entire page is loaded, the screenshot is as follows:

As can be seen from the waterfall flow of requests, similar to Chrome, the browser preloads last.js. Unlike Chrome, Firefox does not preload the image, but waits until other Loading is done after .js is loaded.

In Firefox, the style file does not affect the rendering of the document (the most typical phenomenon is that the web page is displayed in a messy manner at first, without styles, but after the style file is downloaded, it displays normally). In the body, JavaScript Dynamically loaded JS files will block the download of images behind them.

3. In Opera browser

After testing in Opera, I found that the Opera browser is more "observant". All resources are loaded in order, and there is no so-called preloading. Here is a general rendering:

In Opera, style files will block the rendering of the page, which is similar to Chrome. However, from Opera's request waterfall flow, it can be seen that all resources in the page are loaded step by step, and other.js is ahead of last .js loading. No preloading.

Test 2. The style file is at the beginning of the body. The location of the script file is the same as that of test 1.

<br><div class="msgheader">
<div class="right"><span style="CURSOR: pointer" onclick="copycode(getid('phpcode91'));"><u>复制代码</u></span></div>代码如下:</div><div class="msgborder" id="phpcode91">
<br><br> <br> <br>     <title>测试页面</title>
<br> <br> <br>     <link rel="stylesheet" type="text/css" href="example.aspx?sleep=3">
<br>     <div>
<br>         Hi, there!</div>
<br> <br>     <script type="text/javascript"><br />         document.write("<script src='other.aspx?sleep=5'>" + "ipt>");<br />     </script><br> <br>     <div>
<br>         Hi, again!</div>
<br>     <img src="/static/imghwm/default1.png" data-src="images/marx.jpg" class="lazy" alt="Understand the rendering process of html pages in preparation for learning front-end performance optimization (continued)_HTML/Xhtml_Web page production"><br>     <img src="/static/imghwm/default1.png" data-src="images/engels.jpg" class="lazy" alt="Engels"><br>     <img src="/static/imghwm/default1.png" data-src="images/Lenin.jpg" class="lazy" alt="Lenin"><br> <br>     <script src="last.aspx" type="text/javascript"></script><br> <br> <br> <br>
</div><br>经过测试，发现在火狐和Opera中，结果和测试一的一样，而在Chrome中稍微有些不同，在测试一中，图片要等到head中样式文件加载完之后才会下载，但是测试二中会和样式文件并行下载，如下图：

Summary:

Preloading does exist, but it is not found in Opera; Chrome images can be downloaded in parallel with the style files in the body, but not in parallel with the style files in the head. The script is executed after the style file in front of it is loaded. In Chrome and Opera, unloaded resources will block the rendering of the elements behind them, but Firefox will not. Test results may be related to browser version.

After reading this, do you feel a little confused? I want to express it as clearly as possible, but due to my limited level, I can only do this. I hope you can point out the inappropriateness, and you can also do it yourself. Experiment and see.

(End)^_^