Guide: CSS Object Model (CSSOM): Quick Guide
A lot happens between the first HTTP request and the final delivery from a web page. Data transmission and the browser’s rendering pipeline require many different technologies, one of which is the CSS object model, or the CSSOM The CSS object model takes the CSS code and lists each selector in a tree structure for easier parsing. It may not be critical for developers to fully understand this concept, but it is a valuable topic to study if you want to how browsers convert code into a working website In this post, I’m going to discuss the basics of the CSS object model and show you how it works. The term CSS object model describes a map of all CSS selectors and relevant properties for each selectorThese styles can be root elements or have nested children. CSSOM is very similar to the DOM in HTML, which stands for Document Object Model. Both are part of the critical rendering path that’s a series of steps that need to be done display a website correctlyAll these processes happen automatically, Behind the scenes. So why is CSSOM important? It is the card used by the browser render CSS styles correctly on a web page. There is no easy way to tell a computer that all paragraphs in a .main content div must have an extra line height. The solution is the CSS object model that maps all elements and properties of your CSS code. CSSOM makes it easier for the browser to use render styles on the pageThe whole thing is very technical, but it’s worth understanding a little bit about the process, especially if you’re building websites. Both the DOM and the CSSOM are widely used by all web browsers to interpret and display web pages. The diagram below is from Google Developers Web Fundamentals guide, and explains how the DOM is displayed in a web browser IMAGE: Google Developers All information is in both the DOM and CSSOM converted from bytes to digital cards representing every element in a web document. The process works like this: All this happens very quickly and is happening with each individual page requestThis other diagram below shows one example tree structure of the CSSOM IMAGE: Google Developers Note that some properties in the chart have lighter gray font colors. These properties are inherited from the parentSince the body has a specific font size, all elements in the body will also have that font size unless it is overwritten. HTML and CSS strings are converted to tokens what then could be understood as nodes by the browser. These nodes are like objects within the tree structure that determines how the entire page should be constructed. The CSSOM and DOM are complete separate data models, therefore they are parsed separatelyBut they have both similar tree structures, and both serve the same purpose: to structure the browser to display and identify different elements on the page. Since all view happens on the backendyou really don’t need to worry about the CSSOM structure. But it can be helpful to understand how it works. One thing to remember is that the CSSOM must be fully loaded before the web page is displayed because it determines how each element on the page should look. If the page was loaded before the CSSOM, it would appear as plain HTML first, followed by the styles a few seconds later. Browsers specifically avoid that because it would be confusing for end users. And it’s worth noting that the CSSOM cannot be cachedIt must be recreated on every page Actual CSS files can be cached to load the assets faster but display a page in the browser always requires running the CSSOM parserThis also means that JavaScript can have a negative impact on display and performance. I would highly recommend reading this article about external CSS / JS resources and their loading times. The best way to optimize your site is through a natural cascade of resources that are loaded one after the other It is possible to manipulate the CSSOM with JavaScript as it is technically a JS API. But it doesn’t really serve any purpose compared to JavaScript DOM manipulation. The bigger reason to learn more about the CSSOM is to further educate yourself about how websites really work. There are many things we take for granted that keep the internet running smoothly. Understanding a little more about the whole process will help you visualize how it all comes together, and hopefully gain some appreciation for the existence of the World Wide Web. I hope this intro can give you a good idea of what the CSS object model is and how it affects web pages. There there is not much to manipulate in the CSSOM, so it’s a bit different from the DOM. However, it is still a crucial technology in web development and it should clarify the most important aspects of browser display. There are many other resources that discuss the CSSOM and how it works. If you’d like, here are some posts I recommend:
CSS Object Model Overview Exploring the CSSOM: Creating a CSS Object Analyzer What Every Frontend Developer Should Know About Web Page Rendering
CSS Object Model (CSSOM): Quick Guide: benefits
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