Loading order and execution of html, css and js files

I was reading a book today and saw the loading and execution of html, css, and js. I found that I didn’t really understand it. I searched online for half an hour and still couldn’t figure it out. At this time, I found a passage that made me suddenly realize it, as follows ：

HTML page loading and parsing process

1. The user enters the URL (assuming it is an HTML page and it is the first visit), the browser sends a request to the server, and the server returns the HTML file.

2. The browser starts loading the html code and finds that there is a tag in the

tag that refers to an external CSS file.

3. The browser sends a request for the CSS file again, and the server returns the CSS file.

4. The browser continues to load the code of the

part of the html, and the CSS file has been obtained, and the page can be rendered.

5. The browser finds an tag in the code that references an image and sends a request to the server. At this time, the browser will not wait until the image is downloaded, but will continue to render the subsequent code.

6. The server returns the image file. Since the image occupies a certain area and affects the arrangement of subsequent paragraphs, the browser needs to go back and re-render this part of the code.

7. The browser finds a <script> tag containing a line of Javascript code and runs it quickly. </script>

8. The Javascript script executes this statement, which instructs the browser to hide a certain

9. Finally waited for the arrival of , and the browser burst into tears...

10. Wait, it’s not over yet, the user clicked "Skin Change" in the interface " button, Javascript lets the browser change the CSS path of the tag.

11. The browser summoned everyone present, "Everyone pack your bags, we have to start over...", browse The server requests a new CSS file from the server and re-renders the page.

Summary: 1. Generally speaking, it is loaded in the order of html documents

2. It is also best to put the internal or external JS files after all html content, so that It will make users feel that the page loads faster. Otherwise, if all external file (including css and JS) references are placed in

, it means that they must wait until all JS codes are downloaded, parsed and executed. The content of the page cannot begin to be rendered until the browser encounters . This will cause a significant delay in rendering the page, and the browser window during the second delay will be blank.

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