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Preload: Definition, How It Works, Functions & Effects

Last updated: Aug 04, 2023

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What is preload? Have you ever wondered about it? Preload is a performance optimization technique for websites. This technique aims to load important resources before they are requested by the user.

It has the unique characteristic of being able to select important assets and prioritize them for the loading process. Hence, preloading can make the website load faster. In the context of website optimization, page loading time plays a crucial role in providing a good user experience.

Users tend to leave websites that have slow loading times and look for more responsive alternatives. Therefore, speeding up loading times becomes one of the top priorities in website design and development.

For more information about what it is, along with discussions on how it works, its functions, and its effects on Core Web Vitals, please refer to the following guide.

What Is Preload?

Figure 1: Illustration of the Preload Process on a Computer

Preload is a performance optimization technique for websites that aims to load important resources beforehand, even before they are requested by the user.

By doing so, the resources needed to display the website's page will be loaded earlier, resulting in faster loading times.

This technique can be used for various types of resources, such as images, CSS, JavaScript, fonts, and videos.

When a user accesses a website page, the browser will fetch and load various types of resources required, such as images, CSS, JavaScript, and others, following the order from the top to the bottom of the page.

As a result, these resources will be loaded in advance before the browser processes and displays the page.

How Preload Works

Besides understanding what it is, you also need to grasp how this technique works. It basically operates through the following steps:

Identifying important resources: The first step in implementing preload is to identify the resources considered crucial for the website page. These resources include elements required to display the page correctly and ensure an optimal user experience. Examples could be the header (heading tag), CSS files, or JavaScript scripts.
Defining it with the link tag: After knowing which resources to use, you need to define it. This can be done by using the <link> tag in the <head> section of the website page. The tag contains the attribute rel=preload to inform the browser that there are resources to be preloaded.
Specifying the resource type: In the implementation of preload, you will encounter the "as" attribute in the <link> tag, which is used to specify the type of resource. This can be a CSS file, a JavaScript script file, an image, or any other relevant type based on the requirements.
Loading the resource: When the browser encounters the <link> tag with the rel=preload attribute, it initiates the process without waiting for an explicit command to load it. This way, important resources will be loaded in advance before the browser processes and displays the page.
Using the resource: After the important resources have undergone the process, the website page can directly call and utilize these resources. This results in faster loading times.

The Function of Preload

Figure 2: Illustration of Preload CSS Coding Display

Now, you need to understand its functions, especially in website speed optimization. Here are some key functions of you need to know.

1. CSS Resources

Preload is used to load the CSS files required to style and layout the website page in advance, even before the page requests them.

By doing so, the browser can process CSS files faster and display the page with the appropriate style. This, accordingly, helps to improve page loading speed and reduce the time it takes for users to view the page fully.

2. CSS Files

Do you know the function of preloading CSS files? If you are using the critical CSS approach, you need to split the CSS into two parts.

The first part is the critical CSS needed to render the above-the-fold content. Later, the critical CSS is included in the <head> of the document.

Meanwhile, the non-critical CSS will be loaded later using JavaScript. Loading with this approach usually takes time. Therefore, you need to use <link rel=preload> to execute the process.

3. JavaScript Files

The purpose of preloading JavaScript files is to load the files required for interactivity and functionality of the website page in advance, even before the page requests them.

JavaScript files typically contain codes that handle interactive elements, form validation, data processing, or other functionalities required on the website page.

By preloading, the browser can execute these JavaScript codes faster, and the web page can function properly and responsively.

The Effects of Preload on Core Web Vitals

Last but not least, you also need to understand what its effects on Core Web Vitals.

Core Web Vitals are a set of metrics measured by Google to assess the quality and performance of a website page. Here is a complete explanation:

1. Largest Contentful Paint (LCP)

The metric of LCP is useful for measuring the time it takes to display the largest content element, typically an image, video, or text, on the website page.

When you use preloading process, the loading time will appear longer because these elements are loaded first. This makes the page more responsive and provides a better user experience.

2. Cumulative Layout Shift (CLS)

In addition to LCP, you also need to know the effect of this loading technique on CLS. This metric measures the visual stability of the web page during the loading process.

CLS calculates the number of layout shifts that can disrupt the user experience. By using this technique, the impact of unwanted layout shifts can be reduced.

This can happen because the page elements are predicted beforehand and assigned to their correct positions.

3. Responsiveness Metric

The effect of this loading technique on the responsiveness metric can anticipate user interactions while preloading some content that may be needed by the user next.

The responsiveness metric measures how quickly the website page responds to user interactions, such as clicks or swipes.

That concludes the definition, along with discussions about how it works, its functions, and its effects on Core Web Vitals.

By using it effectively, you can improve the loading speed of a website. Search engines like Google consider website speed as one of the factors determining a website's ranking in organic search results.

However, website speed is not the only factor that influences rankings in search engine result pages (SERPs). There are still various other factors, such as domain authority, search intent, and more.

Therefore, relying solely on website speed will not guarantee a top-ranking position on the SERPs. A well-planned SEO strategy is needed to achieve that.

In addition to improving website rankings in search engine results pages (SERPs), an SEO strategy can help you gain quality traffic, online visibility, and customer trust.

If you encounter difficulties in creating a search engine optimization strategy, you can use SEO services by cmlabs to get assistance from our SEO specialists to boost business growth.

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