On the other hand, non-render blocking resources don’t postpone the rendering of the page. After the initial page renders, the browser can safely download them in the background. Let's understand in detail below:
What are Render Blocking Resources?
The process in which the web resources (ex: JS, CSS & HTML files) that are loaded in the webpage potentially cause the browser to block the rendering of the page until they are loaded is called render-blocking resources. These web files can be large, with hundreds of kilobytes of data that must be loaded, in addition to the HTML, before it is rendered.
Therefore, always look for the best solution to eliminate render-blocking resources in above-the-fold content and optimize CSS delivery. Two techniques that are widely used are async & defer loading.
What are Critical and Non-Critical Java Scripts?
What is Inline Delivery?
Inline delivery refers to loading a resource (like JS, CSS) within the HTML code instead of calling/importing that separately.
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Common Things About Async and Defer
Several page speed testing tools are available to analyze a website for site speed and loading time. The most reliable and trusted tools for page speed testing are:
Do Async or Defer Attributes Have Any Effect on Inline Scripts?
No, both async and defer attributes don’t affect inline scripts.
According to Google PageSpeed Insights (PSI) recommendations, your website should deliver critical JS inline and defer all non-critical JS to eliminate render-blocking resources.
What is Defer Parsing?
- First, make parallel requests to fetch the individual files.
- Meanwhile, Continue parsing the document as if it was never interrupted.
- Once the parsing documents' task is finished, Execute each script in the order they were encountered in the document.
Therefore, Defer is very similar to async attribute with just one major differerence.
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The webpage would have to load and execute scripts before rendering the page.
Here’s a usage example of defer attribute of <script>:
<script defer src=”/js/jquery.min.js”> </script>
- Using WordPress Plugins.
- Using the Script method (without using plugins).
- Adding a Code Snippet to the function.php file.
When you add the async attribute to your script tag, the following steps likely will happen.
- It makes parallel requests to fetch the files.
- It continue parsing/loading the document as if it was never interrupted.
- Execute the individual scripts the moment the Java files are downloaded.
What is the difference Between Async and Defer Loading?
Async and defer are similar in allowing scripts to load without blocking the HTML parsing. It means users see page content more quickly. But they do have differences in eliminating render-blocking resources.
2. Scripts loaded with async are parsed and executed immediately when downloading the resource. Scripts with defer attributee don’t execute until the HTML doc is parsed.
3. Async scripts may load out-of-order, but defer scripts are executed in the order they appear in the markup.
4. Async follows Load-first order. Here the order of documents doesn’t matter. It may load and execute while the document has not yet been fully downloaded. It also happens if scripts are small or cached and the document is long enough. Defer follows Document order (as they go in the document). It executes after the document is loaded and parsed (they wait if needed), right before DOM Content Loaded.
Making these changes makes the code easier to read and reduces the file size, thus shaving a few hundred milliseconds or even seconds off the loading time.
Access your WordPress dashboard and navigate to the Plugins tab to install this plugin. Then, select the Add New option on the window's upper side. Afterwards, you can look for Autoptimize or other plugins in the search bar. Click Install Now, activate it for the website you want, and you are ready.
W3 Total Cache
What is interesting about this plugin is that it incorporates multiple extra features for optimizing WordPress. Caching represents storing specific files on a user’s computer to improve his website experience. Subsequence visits will be more accessible, and the load times will improve.
Key Features of W3 Total Cache:
- Defer offscreen images using Lazy Load to improve the user experience.
- Browser caching using cache-control, future expire headers, and entity tags (ETag) with “cache-busting”.
Speed Booster Pack
With its ever-evolving codebase, Speed Booster Pack aims to increase your site performance, page load times, and PageSpeed (Lighthouse) scores! Optimocha, the company behind Speed Booster Pack, has a team with a pathological obsession for speed, so you can be sure your site will be in good hands.
Significant Features of JCH Optimize:
- GZip compresses the combined files and generates sprites to combine background images.
- This plugin can exclude files from combining to resolve conflicts.
- You will get optimized CSS delivery to eliminate render-blocking.
- CDN and cookie-less domain support and lazy load images.
Which is Better, Async or Defer?
Whereas with the defer attribute, the file gets downloaded asynchronously. But executed only when the document loading is completed. Also, scripts will perform in the same order as they are called.
Therefore, defer is the correct attribute choice when a script depends on another and defer is the right choice when async is not an option.
Why We Prefer to Defer Over Async?
Defer scripts can never block synchronous scripts. Whereas async scripts depend on how quickly they download. Synchronous scripts are typically made synchronous because they are essential for the web page's critical content.
Therefore, it’s always better to use defer attribute so synchronous scripts are not blocked from executing. Also, their critical work is completed more quickly.
FAQs About Eliminate Render-Blocking Resources With Async and Defer:
Are images render blocking?
Remember, images aren't rendered blocking, so if you have images on the blue DOM line, you can safely ignore those, although you will still want to optimize your images.
What is inline CSS?
Inline CSS allows you to apply a unique style to one HTML element at a time. You assign CSS to a specific HTML element using the style attribute with any CSS properties defined within it.
Conclusion: Eliminate Render-Blocking Resources With Async and Defer:
So, we have discussed the best strategies to eliminate render-blocking resources.
To help visitors load the visible portion of your page more quickly, you should delay loading resources that aren’t immediately required.