preload improves web page speed by loading key resources in advance, suitable for core files on the first screen. ① Used to prioritize the loading of key CSS, JS, fonts, images, etc.; ② Use the as attribute correctly to specify resource types such as script, style, font, and image; ③ Pay attention to avoid duplicate loading, testing loading timelines, and handling cross-domain and compatibility issues.
The loading speed of web pages directly affects user experience and search engine rankings, and resource preloading is one of the important means to optimize loading performance. HTML5 provides the feature <link rel="preload">
, which can actively tell the browser to load key resources in advance, such as fonts, scripts, style sheets, etc., thereby speeding up page rendering.

Let’s take a look at how to use HTML5 preload to improve performance and some things to pay attention to.

What is preload? What is its use?
<link rel="preload">
is a declarative way to enable browsers to start loading specified resources in the early stages of parsing HTML. Compared to traditional loading methods, it triggers downloads earlier to avoid delays caused by script or style blocking.
For example, if you want to load a custom font or a key JavaScript file, you can write it in like this:

<link rel="preload" href="font.woff2" as="font" type="font/woff2" crossorigin>
In this way, the browser knows that this resource is very important and will be loaded first.
Which resources are suitable for preload?
Not all resources are suitable for preloading, and choosing the wrong one will affect performance. The following types of resources are more suitable for using preload
:
- Key CSS and JS : Core scripts and styles for first-screen rendering.
- Font files : Especially custom fonts, which are usually large in size, can reduce text flashing as soon as possible.
- Pictures and videos : Especially the large image or video cover that must be displayed on the first screen.
- Web Workers or WASM files : Preloading can also be considered if the project relies on these technologies.
? Tips: Use preload only for resources on "critical paths", otherwise it may occupy bandwidth and affect other resource loading.
How to use preload correctly?
It is very important to use as
attribute correctly, which helps the browser to determine how to handle this resource. Common values ??include script
, style
, font
, image
, etc.
To give a few examples:
<!-- Preload script--> <link rel="preload" href="main.js" as="script"> <!-- Preload style--> <link rel="preload" href="styles.css" as="style"> <!-- Preloaded picture--> <link rel="preload" href="hero.jpg" as="image"> <!-- Preload fonts (note crossOrigin) --> <link rel="preload" href="myfont.woff2" as="font" type="font/woff2" crossorigin>
A few things to note:
- Use
crossorigin
to ensure that cross-domain resources such as fonts can be cached and reused. - Don't abuse it, otherwise it will cause resource competition.
- During testing, you can use the browser developer tool to view the resource loading timeline to confirm whether it has actually loaded in advance.
Probable problems
Although preload is useful, there are some pitfalls to be paid attention to:
- Repeat loading : If you have introduced a resource through other methods (such as
<script src="..."></script>
), preloading it once will be unnecessary. - Unsupported types : Some old browsers may not recognize preload and need to be compatible.
- Cache policy affects loading behavior : if the resource is already in the cache, preload may not be requested again, but will still trigger the load completion event.
In general, HTML5's preload is a simple but effective method of performance optimization. As long as it is used reasonably, it can significantly improve the page loading experience. Basically that's it, don't be too complicated but don't ignore the details.
The above is the detailed content of Preloading Resources for Performance using HTML5 Preload Hint. For more information, please follow other related articles on the PHP Chinese website!

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