• author: developedbyed

Understanding Render Blocking and Its Impact on Website Performance

In today's fast-paced digital world, having a website that loads quickly has become a vital part of providing a seamless user experience. One common factor that can significantly affect the loading speed of a website is render blocking. In this article, we will delve into what render blocking is, why it is important to address, and how different elements impact the rendering process.

What is Render Blocking?

When a user visits a website, their browser needs to download and parse the HTML document to display the content. However, certain elements within the HTML document, such as external resources like fonts or JavaScript files, can block the rendering process. This means that the browser has to wait until these resources are downloaded and processed before it can continue rendering the rest of the website.

During the rendering process, the browser builds a waterfall-like structure in the browser's network tab, indicating the order in which different resources are loaded. It is not uncommon to see different-colored blocks within this waterfall, each representing a specific resource.

Impact of Render Blocking on Website Performance

Render blocking can have a significant impact on the loading speed of a website. The more resources that need to be downloaded and processed, the longer it will take for the website to render fully. This delay can result in a poor user experience, particularly for visitors on slower internet connections.

To illustrate this, let's consider a simple HTML document that we will use as an example. When we load this document, we can see in the network tab that only the HTML file itself is being loaded, and no other resources are fetched. This is because the rendering pipeline is not being blocked by any external assets.

However, by adding additional resources such as images, stylesheets, or scripts, we can observe how they impact the rendering process. It is important to note that while images do not block rendering, stylesheets and scripts can cause rendering delays.

Stylesheets and Their Impact

Stylesheets play a crucial role in defining the visual appearance of a website. Traditionally, stylesheets are included in the HTML document using the <link> tag. This approach is effective in maintaining clean HTML files and allows for modularity and separation of stylesheets based on different pages or media types.

However, it is worth noting that including stylesheets with the <link> tag can block the rendering process. While the impact may vary based on the size and complexity of the stylesheet, it is generally a good practice to keep stylesheets concise and optimized. Large stylesheets can significantly slow down the rendering process, leading to delayed page rendering and a poorer user experience.

Alternatively, stylesheets can also be included inline within the <head> tag. This approach eliminates render blocking, as the styles are applied without any external file dependencies. This practice can be particularly useful for loading critical CSS, which refers to the styles necessary for the initial view of the webpage.

Modern frameworks such as Tailwind CSS offer an efficient way to write concise and performant CSS. Tailwind CSS utilizes utility classes, which are small CSS snippets applied directly to HTML elements. By using Tailwind CSS, developers can minimize the size of their stylesheets without sacrificing design flexibility.

JavaScript and Its Impact

JavaScript plays a significant role in adding interactivity and dynamic functionality to websites. However, like stylesheets, including JavaScript files can also lead to render blocking. By default, when the browser encounters a <script> tag, it pauses HTML parsing and downloads the JavaScript file. Once the file is downloaded and parsed, the browser resumes parsing the rest of the HTML.

To minimize render blocking caused by JavaScript, two attributes can be added to the <script> tag: defer and async.

  • Defer: Adding the defer attribute to a <script> tag defers JavaScript execution until after the entire HTML document has been parsed. This allows the HTML rendering process to continue uninterrupted. The browser automatically starts downloading the JavaScript file in the background while rendering the HTML. Once the HTML parsing is complete, the browser executes the JavaScript file.

  • Async: On the other hand, adding the async attribute starts executing the JavaScript file as soon as it is available. The execution does not wait for the HTML parsing to finish. This attribute is useful when the JavaScript code does not rely heavily on the DOM and can be executed independently.

The choice between defer and async depends on the specific use case. If the JavaScript code relies on the complete HTML structure or other resources, using defer ensures the document is fully parsed before executing the code. If the JavaScript code is decoupled from the DOM or other resources, async can be employed for faster execution.

Optimizing Font Loading

Fonts are an essential aspect of website design, but adding fonts from external sources can introduce render blocking. Instead of relying on content delivery networks (CDNs) like Google Fonts, it is often more efficient to download the fonts locally and import them into the project.

By hosting fonts locally, a website can minimize external dependencies and reduce the risk of potential delays caused by network issues or server downtimes. Importing fonts directly from the project files ensures faster loading times and a more consistent user experience.

Frameworks and Render Blocking

Modern web frameworks take several measures to address render blocking and optimize website performance automatically. These frameworks employ code splitting techniques, which involve breaking down large JavaScript files into smaller, more manageable chunks. By dynamically loading only the necessary code, frameworks can improve rendering times and prioritize critical components.

Additionally, frameworks often include built-in optimizations for CSS, such as minification and tree-shaking, to eliminate render blocking caused by stylesheets. These optimizations help reduce file sizes and ensure faster rendering.

While frameworks handle many render blocking issues, it is still valuable for developers to have an understanding of the underlying concepts to further optimize their websites.


Render blocking can have a significant impact on the loading speed and user experience of a website. By understanding the various elements that can block rendering, developers can apply best practices to minimize delays and optimize performance.

Considering the impact of stylesheets, JavaScript, and fonts on render blocking, developers can employ techniques such as inline styles, code splitting, and local font hosting to alleviate these issues. By choosing the appropriate attributes, such as defer or async, for JavaScript files, developers can balance rendering priorities and maintain a smooth user experience.

While modern frameworks handle many render blocking aspects automatically, having a fundamental knowledge of the rendering process allows developers to fine-tune their websites and ensure optimal performance. It is crucial to strike a balance between aesthetics and performance to create fast-loading and visually appealing websites.

So, whether you are a seasoned developer or a beginner, understanding render blocking can help you build websites that load quickly and provide an outstanding user experience. Stay informed, experiment with different approaches, and optimize your websites accordingly.

Previous Post

Building a Fun Project on Astro: Exploring the Basics

Next Post

Creating a React Portfolio Website with Codex Visual IDE

About The auther

New Posts

Popular Post