Along with more page speed tips, get up to speed with the fundamentals of website performance optimization:Full Guide
What Is Render Blocking?
If you've run a website loading speed test like Google's PageSpeed Insights, you may have seen an opportunity noted in the test results like "Eliminate render-blocking resources". Avoiding or eliminating render blocking resources altogether is an important strategy to maximize website speed and ace speed tests.
To make the needed changes and get the best results, it can help to take a closer look at render blocking and why it matters.
<script> references are considered render blocking because the process of otherwise loading and displaying the page is paused until those files are downloaded and interpreted by the browser.
The result is a delay in the display of the page to the user. Eliminating render blocking resources minimizes this delay so the page can load and display content as soon as possible.
Critical vs Non-Critical Resources
Get the full scoop about how render blocking impacts the loading process and how to structure the way resources are loaded to avoid a negative impact on page speed:Loading Structure
Lean = Fast
The fastest loading files are the files that don't load at all.
Before putting any of these strategies into practice, it's a good idea to carefully review any included resources and make some shrewd decisions about what should and shouldn't be included.
This is often best done by taking a close look at each content element, feature and included resource and asking yourself, "Is this good for the user?"
Underused or superfluous features, plugins and other files can be good candidates for removal or adjustment. For example, while banner-style image rotators can seem like a good idea, if most users scroll past before seeing more than one or two slides, it may not be worth the extra resources. Instead, a single static image with a clear, focused headline may deliver a stronger and more effective message, with less page weight.
Custom fonts can also be a good source of page weight reduction and even an opportunity to refine the design. Custom font systems like Google Fonts make it easy to add stylish and eye-catching text styles and it can be tempting to use them too liberally - at the expense of both tasteful design and page speed. For most applications, 4-6 custom font variants yields a good balance of design style and page speed.
For the resources that remain after a prudent spring cleaning, the strategies below will help limit or fully eliminate the render blocking impact during the loading process.
Loading CSS styling for above-the-fold content before the page is displayed avoids the unpleasant potential user experience of seeing unstyled content before the CSS is applied - sometimes called a flash of unstyled content.
So how do you eliminate render blocking CSS if it's necessary to properly display the page?
The answer is to separate critical CSS from non-critical CSS.
- The most critical CSS styling is loaded and applied as soon as possible with an intentionally render blocking conventional reference to a very small CSS file (<10KB) or with an inline
<style>block embedded directly in the HTML.
- Non-critical CSS is then loaded in the background without render blocking using a technique often called asynchronous CSS or async CSS.
A typical setup for critical and non-critical CSS looks like this:
Don't let render blocking CSS resources slow down page speed. Explore strategies to load low priority, non-critical CSS in the background and get the finer points of async CSS techniques:Asynchronous CSS
async attribute can be a good option as well, but may interrupt rendering to execute.
Learn the difference between
defer, how to deal with inline
<script>s that are dependent on
defer'd resources, plus other tips to streamline the loading process:
Custom, stylish fonts are an easy way to add visual interest, establish branding and give your website a unique look, and Google Fonts makes it as easy as adding one line of code to include one or more fonts to the page.
That one line is a
<link> reference to a CSS file, which by nature is render blocking. This means that the render blocking effect of Google Fonts can be eliminated with the same asynchronous CSS technique that's commonly used for other non-critical or low-priority CSS resources.
But don't stop there! Combine async CSS loading with the
preload resource hints to speed up Google Fonts even more:
Whether you prefer to use the conventional asynchronous snippet or the Global Site Tag version of the tracking code, Google Analytics can also be reconfigured to avoid render blocking and minimize the impact on page speed.
defer attribute on the external
defer'd external file for the same effect.
Eliminate the render blocking effect of Google Analytics with two simple steps:Defer Google Analytics
defer attribute to load the file in the background while the page continues to load.
It's generally good practice to move all inline
<script>s rely on a
defer'd file like jQuery, the inline content can be wrapped in an appropriate event listener:
DOMContentLoaded event and after any
defer'd files they may depend on like jQuery are executed and ready.
Refine the loading process even further with strategies like asynchronous CSS and resources hints and other tips to improve page speed. Get started with an overview of HTML structure and how to configure the loading process for speed:Loading Structure