Use Cases
Understanding Speculation Rules API in Modern Browsers


5 mins read

Understanding Speculation Rules API in Modern Browsers

Shubham Saurabh
Jun 17, 2024
Understanding Speculation Rules API in Modern Browsers

The web is continuously evolving to offer faster and more efficient browsing experiences. One of the latest advancements contributing to this evolution is the Speculation Rules API. This API empowers developers to optimize their websites by predicting and preloading resources that users are likely to need. This article delves into the intricacies of the Speculation Rules API, exploring its purpose, functionality, and practical implementation.

What is the Speculation Rules API?

The Speculation Rules API is a set of browser APIs designed to improve web performance by allowing developers to specify rules for preloading and prefetching resources. By utilizing this API, browsers can make educated guesses about which resources a user will need next and load them in advance. This proactive loading can significantly reduce latency, providing a smoother and faster user experience.

How Does the Speculation Rules API Work?

The Speculation Rules API operates by enabling developers to define rules for resource preloading based on user behavior patterns. These rules guide the browser in deciding which resources to fetch ahead of time, even before the user explicitly requests them. The key components of this API include:

  1. SpeculationRules Interface: This is the primary interface for defining and managing speculation rules. It allows developers to create rules that specify which resources should be preloaded or prefetched based on certain conditions.
  2. Link Types: The API supports various link types such as "prefetch" and "preload". Prefetching involves loading resources that might be needed soon, while preloading focuses on resources that are critical for the current page.
  3. Conditions: Developers can set conditions under which resources should be preloaded. These conditions can be based on user interactions, navigation patterns, or specific elements on the page.

Practical Implementation

To implement the Speculation Rules API, developers can follow these steps:

  • Define Speculation Rules: Use the SpeculationRules interface to create rules that specify which resources to preload or prefetch.
const speculationRules = new SpeculationRules();
  type: 'prefetch',
  url: '/path/to/resource',
  condition: 'always'
  • Attach Rules to the Document: Attach the defined rules to the document to instruct the browser on how to handle preloading.
document.speculationRules = speculationRules;
  • Monitor and Adjust: Continuously monitor the performance of preloaded resources and adjust the rules as needed to optimize efficiency.

This how a typical speculation looks like 

<script type="speculationrules">
    "prerender": [
        "where": {
          "and": [
            { "href_matches": "/*" },
            { "not": { "href_matches": "/logout" } },
            { "not": { "href_matches": "/\?(^|&)add-to-cart=*" } },
            { "not": { "selector_matches": ".no-prerender" } },
            { "not": { "selector_matches": "[rel~=nofollow]" } }
    "prefetch": [
        "urls": ["next.html", "next2.html"],
        "requires": ["anonymous-client-ip-when-cross-origin"],
        "referrer_policy": "no-referrer"

Example Use Case

Consider a website with a multi-step form. By using the Speculation Rules API, developers can preload resources for subsequent steps based on user interactions with the current step. This approach ensures that each step loads instantly, providing a smooth and uninterrupted experience.

const formSpeculationRules = new SpeculationRules();

formSpeculationRules.addRule({ type: 'preload', url: '/form-step-2.js', condition: 'document.querySelector("#step1").value !== ""' });

document.speculationRules = formSpeculationRules;

In this example, the script for step 2 of the form is preloaded as soon as the user completes step 1, reducing wait times and enhancing the overall user experience.

Benefits of Using the Speculation Rules API

Implementing the Speculation Rules API in web applications offers several advantages:

  • Proactive Resource Loading: By preloading resources that are likely to be needed, the Speculation Rules API reduces the wait time for users, leading to faster page loads.
  • Improved User Experience: Faster load times and smoother transitions between pages enhance the overall user experience, keeping visitors engaged and satisfied.
  • Efficient Resource Management: The API helps in managing resources more efficiently by prioritizing critical assets, ensuring they are available when needed without unnecessary delays.
  • Optimized Bandwidth Usage: By loading only the necessary resources ahead of time, the API can optimize bandwidth usage, particularly beneficial for users on slower or limited networks.
  • Seamless Navigation: With critical resources preloaded, navigation through web applications becomes seamless, reducing the chances of users abandoning the site due to slow load times.
  • Higher Conversion Rates: Improved performance and user experience can lead to higher conversion rates, as users are more likely to complete desired actions on a fast, responsive website.
  • Flexible Conditions: Developers can set specific conditions for preloading resources, allowing for a highly customizable and context-aware resource management strategy.
  • Predictive Loading: By analyzing user behavior patterns, the API can make informed decisions on which resources to preload, tailoring the experience to individual users.

Drawbacks of Speculation Rules

While Speculation Rules offer many advantages, there are also some potential drawbacks to consider.

Here are some potential drawbacks of Speculation Rules:

  • Learning Curve: Implementing the Speculation Rules API requires a good understanding of the API and the specific needs of the web application, which can be a steep learning curve for some developers.
  • Maintenance: Continuously monitoring and adjusting preloading rules to match changing user behavior and content can be time-consuming and requires ongoing attention.
  • Increased Initial Load: While preloading can reduce latency for future actions, it can increase the initial load time and resource usage, especially if not managed properly.
  • Potential Waste: If preloaded resources are not used by the user, it can lead to wasted bandwidth and processing power, particularly on resource-constrained devices.
  • Limited Support: As a relatively new technology, the Speculation Rules API may not be supported by all browsers or browser versions, limiting its effectiveness across different user bases.
  • Inconsistent Behavior: Variations in how different browsers implement the API can lead to inconsistent behavior and performance across platforms.
  • Preloading Sensitive Data: If not handled carefully, preloading can inadvertently expose sensitive data by loading resources that may contain private information before the user explicitly requests them.
  • User Tracking: The predictive nature of the API could raise privacy concerns, as it involves tracking user behavior to make informed preloading decisions.


The Speculation Rules API offers significant benefits in terms of performance optimization and user experience enhancement. However, it also comes with challenges that developers need to consider carefully. By understanding both the advantages and potential drawbacks, developers can implement the Speculation Rules API effectively to create fast, responsive, and user-friendly web applications. You can read more from MDN here, https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API


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About the Author
Shubham Saurabh
Founder & CEO at Auditzy
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