Optimizing web performance and critical rendering path

Learn about the impact that CSS has on website performance

Performance is a very important aspect of every web application.

Web page should be loaded as quickly as possible and the animation should flow smoothly.

People are very sensitive to any kind of motion. For any kind of visual change on the screen, like scrolling, hover effect, opening the sidebar etc., the browser is going to render a new frame onto the screen. When rendering of the frame takes longer, the frame rate drops, and it negatively impacts the user experience.

Low frame rate will create a poor user experience. For this to happen, there doesn’t need to be some complex animation, it can happen with just a regular page scrolling!

I am pretty sure that you already had experience with websites that have a low frame rate, which causes that “laggy” animation? You click on something, open the sidebar and the whole page is lagging. Or even worse, you might have noticed that you have a laggy experience when scrolling the web page. Scrolling doesn't seem like something you had to think about from the performance perspective, but you should. Therefore, in this course we will be talking about fixing the performance issues and so, you might expect that this course is about JavaScript…but surprisingly – it’s not. This course is about CSS. A lot of developers think that as long as JavaScript runs well, there's nothing else left to do for the performance optimization. JavaScript has a great impact on the performance of the website, however CSS is unjustly neglectred in this area. In many situations CSS might be the only problem.

There’s another very important impact that CSS has on the webpage.

CSS is render blocking (by default). The browser will not begin to render the page until all CSS has been downloaded. Any delays on the Critical Rendering Path will leave users looking at a blank screen. CSS is truly critical if you want to have a fast loading page. Sometimes, just to parse CSS file it takes more than 500ms. Just to parse, without network – just a single CSS loaded from a hard drive will cause users to see only a blank screen until the parsing process is finished.

Optimizing the performance your website may also have an impact on the google search result ranking algorithm.

In this course we will show you CSS in a new light.