What Is Render Blocking?
Render blocking refers to the files and resources that delay a webpage from appearing on the screen. These files, typically JavaScript and CSS, must be fully loaded and processed by the browser before the page’s content can be displayed to the user.
Rendering itself is the process wherein the browser converts HTML, CSS, and JavaScript code into the visual content you see on a webpage. When certain resources hold up this process, we say the rendering is blocked.
A file that blocks the page’s rendering is referred to as a render blocking resource. When the browser encounters such a resource, it may pause rendering until that file is fully downloaded and processed. During this time, other parts of the page will be delayed from appearing, even if they are already loaded.
Common render blocking resources include synchronous JavaScript, CSS files that are not inlined or deferred, and external fonts that lack proper loading strategies.
In this article, we’ll cover:
Importance of Render Blocking Resources
Some render blocking resources contain critical data that the browser needs to properly display the webpage. For example, CSS files define how elements should appear to users. So, it includes code that specifies the colors, fonts, and layouts to be used on the webpage.
This makes the CSS file crucial for the browser to display the page correctly. However, when the CSS file blocks rendering, the page will appear unstyled or blank until the CSS file is correctly loaded and rendered.
Overall, you do not want render blocking resources on your site because of the impact they can have on your SEO and usability.
For one, they slow down your page load time, which negatively impacts user experience and page experience metrics, such as largest contentful paint (LCP). These will, in turn, affect your rankings and conversions, as they will cause your users to leave your site.
How Rendering Affects SEO and User Experience
Render blocking directly affects the Kernwebvitalen, which are a set of three metrics that Google uses to evaluate your page experience, which is a key rangschikkingsfactor.
Specifically, they can affect the largest contentful paint (LCP), which Google uses to evaluate how long it takes the largest element on the webpage to display on the user’s screen.
Render blocking resources also impact other metrics, such as first contentful paint (FCP), which measures how quickly content becomes visible to users. While the first contentful paint is not a Core Web Vitals metric, it is measured by the Core Web Vitals tool and is valuable for evaluating the largest contentful paint.
Render blocking also causes a noticeable drop in user satisfaction as visitors are less likely to wait on pages that take too long to display content. This results in higher bounce rates and lower engagement and conversion rates.
Common Render Blocking Resources
Certain files, especially CSS, JavaScript, and font files, are known to block rendering under specific conditions. These code files play a direct role in how the page is structured and displayed, so the browser often delays rendering until they are fully processed.
1 CSS Files
CSS files are a significant contributor to render blocking. These files contain crucial information about the page’s styling and layout, which makes them essential for usability and user experience.
Therefore, the browser must fully download and process them before it can begin painting any part of the page. Otherwise, the browser would not know how to style the elements.
The length of the delay will vary depending on the size and number of CSS files the browser needs to load. The more CSS files a page uses, or the larger the file size, the longer this delay can be, especially on slower networks.
2 JavaScript Files
The browser may block rendering when it encounters JavaScript files, especially those that modify the DOM or interact with CSS, until they are fully downloaded and executed.
These JavaScript files are used for social media widgets, analytics trackers, and advertisements, and can be hosted either locally or on third-party servers.
3 Font Files
Many websites load their fonts from third-party sources, such as Google Fonts or Adobe Fonts. These font files can cause delays in text rendering when they are not optimized or if the hosting server is slow.
As a result, users may not see any text at all until the font finishes loading. This situation is known as invisible text or flash of invisible text (FOIT). Developers can prevent this by using fallback fonts that display until the desired font is loaded.
Render Blocking Best Practices
Developers can implement specific optimization techniques to minimize the occurrence and impact of render blocking resources. Below are some common methods.
1 Inline Critical CSS
Insert critical CSS directly into the <head> tag of the page’s HTML code. Specifically, you should focus on the CSS code that is crucial for the elements displayed above the fold.
This ensures the browser can apply essential styling immediately without waiting to download external stylesheets. Once the critical CSS is in place, the browser can then load the remaining styles from the external file.
2 Use Media Queries or Preload for CSS
Large CSS files can block rendering, as the browser must wait for the entire file to load before it can begin rendering. To prevent this, ensure that you split your CSS files into smaller files. Once done, include media queries media attribute to load them conditionally.
Media queries are a CSS feature that enables developers to apply distinct CSS styles based on the user’s device, such as their screen size, resolution, or orientation. This is crucial for responsive design, as it ensures each device displays only the styles relevant to it.
Additionally, developers can add a preload resource hint <link rel="preload"> to important CSS files. Preload ensures that the browser identifies such elements quickly and loads them early, making them easily accessible once needed.
3 Defer or Async Non-Critical JavaScript
De defer attribute tells the browser to download a script in the background and execute it only after the HTML has been fully parsed. On the other hand, asynchrone downloads the script in parallel and executes it immediately once it is ready, possibly before parsing is complete.
Be sure to include either the defer of asynchrone attribuut in de <script> tag of non-critical JavaScript, particularly those that do not affect the page’s initial appearance. This ensures they do not block rendering and affect your SEO.
4 Reduce Unnecessary Plugins, Widgets, and Scripts
Unnecessary plugins, widgets, and scripts can block the rendering process. Many of them, whether hosted on your server or third-party servers, load their own JavaScript and CSS files on every page, even when they are not needed.
This increases page load time, consumes resources, and can block the rendering process. However, you can avoid this by auditing your site regularly to remove plugins, widgets, and scripts that do not provide real value.
5 Install Performance Plugins
Performance plugins, such as WP Rocket, can implement performance improvements by deferring JavaScript, inlining critical CSS, and delaying the loading of third-party JavaScript files until they are needed.
These optimizations help reduce render blocking, improve Core Web Vitals scores, and speed up the time it takes for a webpage to become visually ready and interactive for users. So, install WP Rocket on your site to enhance your SEO, improve user experience, and prevent render blocking issues.