Swift Performance can generate the Critical CSS (or Critical Path) on the fly for every page.
You can use this feature by going to Settings->Optimizations->Styles: Generate Critical CSS. If you would like to add some custom CSS rules to Critical CSS you can add them in Extra Critical CSS.
What is Critical CSS?
A request for a CSS file can significantly increase the time it takes a web page to render. The reason is that by default the browser will delay page rendering until it has finished loading, parsing and executing all the CSS files referenced in the
<head> of your page. It does this because it needs to calculate the layout of the page.
Unfortunately, this means that if we have a really large CSS file and it takes a while to download, our users will end up waiting until the whole file has been downloaded before the browser can begin rendering the page. Fortunately, there is a sneaky technique that allows us to optimize the delivery of our CSS and mitigate the blocking. This technique is known as optimizing the critical rendering path.
The critical rendering path represents the steps that the browser takes to render a page. We want to find the minimum set of blocking CSS, or the critical CSS, that we need to make the page appear to the user. A critical resource is any resource that may block the initial rendering of the page. The idea behind this is that the website should get the first screen’s worth of content (or “above-the-fold” content) to the user in the first few TCP packets of response. To give you a brief idea of how this would work for a web page, take a look at the image below.
In the example above, the critical part of the web page is only what the user can see when they first load the page. This means that we only need to load the minimum amount of CSS required to render the top portion of the page across all breakpoints. For the remainder of the CSS, we don’t need to worry as we can load it asynchronously.
How do we determine what is considered critical CSS? Determining the critical CSS for a page is rather complex and requires you to walk through the web page’s DOM. Next, we need to determine the list of styles that currently apply to each element in view. Doing this manually would be a tedious process, but Swift´s Generate Critical CSS will do this all automatically.