On the other hand, non-render blocking resources don’t postpone the rendering of the page. The browser can safely download them in the background after the initial page render.
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 load a resource (ex: like JS, CSS) within the HTML code instead of calling/importing that separately.
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Common Things About Async and Defer
There are several page speed testing tools 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 & defer attributes don’t have any effect on 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 as well as 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 finishing to render 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 as well as 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 & Defer Loading?
Async and defer 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 the resource is done downloading. Whereas scripts with defer attributee don’t execute until the HTML doc is done being parsed.
3. Async scripts may load out-of-order, whereas defer scripts are executed in the order in which they appear in the markup.
4. Async follows Load-first order. Here the order of document 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. whereas 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.
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 critical content on the webpage.
Therefore, it’s always better to use defer, 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 execute in the same order as they are called.
Therefore, defer is the right choice of attribute when a script depends on another script. And, defer is the right choice when async is not an option.
FAQs: 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 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.