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 is 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 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:
Does 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.
2. It consequently helps to speed up your website.
3. It gives a better user experience to your website visitors.
4. It makes your site load faster
5. It helps improve your search ranking on desktop and mobile.
6. It helps improve the performance of websites.
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 function.php file.
3. It consequently helps to speed up your website.
4. It gives a better user experience to visitors of websites.
5. It makes your website load faster.
6. It helps to improve your search ranking on desktop and mobile.
7. It helps to improve the performance of websites.
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.
Difference Between Async and Defer Loading?
Async and defer are similar in that they allow scripts to load without blocking the HTML parsing. It means users see page content more quickly. But they do have differences in eliminate render-blocking resources.
2. Scripts loaded with async are parsed and executed immediately when downloading the resource. Whereas 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.
To install this plugin, you simply need to access your WordPress dashboard and navigate the Plugins tab. Then, select the Add New option located on the upper side of the window. After that, you can look for Autoptimize or other plugins in the search bar. Click on Install Now, active it for the website you want, and you are good to go.
W3 Total Cache
What is interesting about this plugin is that it incorporates multiple extra features for optimizing WordPress. Caching represents the process in which specific files are stored on a user’s computer to make his experience with the website better. Subsequence visits will be more accessible, and the load times will improve.
- Compatible with shared hosting, virtual private / dedicated servers and dedicated servers / clusters
- Transparent content delivery network (CDN) management with Media Library, theme files and WordPress itself
- Mobile support: respective caching of pages by referrer or groups of user agents including theme switching for groups of referrers or user agents
- Accelerated Mobile Pages (AMP) support
- Secure Socket Layer (SSL/TLS) support
- Caching of (minified and compressed) pages and posts in memory or on disk or on (FSD) CDN (by user agent group)
- Caching of feeds (site, categories, tags, comments, search results) in memory or on disk or on CDN
- Caching of search results pages (i.e. URIs with query string variables) in memory or on disk
- Caching of database objects in memory or on disk
- Caching of objects in memory or on disk
- Caching of fragments in memory or on disk
- Caching methods include local Disk, Redis, Memcached, APC, APCu, eAccelerator, XCache, and WinCache
- Minification of posts and pages and RSS feeds
- Minification of inline, embedded or 3rd party CSS with automated updates to assets
- 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”
- Import post attachments directly into the Media Library (and CDN)
- Leverage our multiple CDN integrations to optimize images
- WP-CLI support for cache purging, query string updating and more
- Various security features to help ensure website safety
- Caching statistics for performance insights of any enabled feature
- Extension framework for customization or extensibility for Cloudflare, WPML and much more
- Reverse proxy integration via Nginx or Varnish
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 that your site will be in good hands.
- Page Cache
- Optimize CSS/JS/HTML
- GZip compress the combined files.
- Generate sprite to combine background images.
- Ability to exclude files from combining to resolve conflicts
- Optimize CSS Delivery to eliminate render blocking
- CDN/Cookie-less Domain support
- Lazy load images
Why We Prefer 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 important 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.
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.
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.
- Don’t use CSS imports.
- Load conditional CSS with media attributes
- Try to load all custom fonts locally
To help visitors load the visible portion of your page more quickly, you should delay loading resources that aren’t immediately required.