- #Javascript run as soon as element renders series#
- #Javascript run as soon as element renders download#
It describes all of the CSS selectors on the page, their hierarchy and their properties. Similar to HTML files and the DOM, when CSS files are loaded they must be parsed and converted to a tree - this time the CSSOM. The CSSOM, together with the DOM, to build the render tree, which is in turn used by the browser to layout and paint the web page.
#Javascript run as soon as element renders series#
Both of them are part of the critical rendering path which is a series of steps that must happen to properly render a website. The CSSOM is very similar to the Document Object Model (DOM). The CSS Object Model (CSSOM) is a map of all CSS selectors and relevant properties for each selector in the form of a tree, with a root node, sibling, descendant, child, and other relationship. You may well have heard of the DOM before, but have you heard of the CSSOM (CSS Object Model)? Before I started researching this topic a little while back, I hadn't!
In order to mark a resource as important and therefore more likely to be downloaded early in the rendering process, a link tag with rel="preload" can be used. The manner in which they do this varies between different browsers so cannot be relied upon to behave a certain way.
#Javascript run as soon as element renders download#
Preloading resourcesĪs an aside, modern browsers will continue to scan the HTML whilst blocked and 'look ahead' to what external resources are coming up and then download them speculatively. Īsync means that the file will be executed as soon as it loads, which could be during or after the parsing process, and therefore the order in which async scripts are executed cannot be guaranteed. If multiple files have the defer attribute, they will be executed in the order that they were discovered in the HTML. More on that in a bit too, but in summary:ĭefer means that the execution of the file will be delayed until the parsing of the document is complete. Both allow the parser to continue whilst the JavaScript file is loaded in the background, but they operate differently in the way that they execute. There are two attributes that can be added to script tags to mitigate this: defer and async. JavaScript files are a little different - by default they block parsing of the HTML whilst the JavaScript file is loaded and then parsed. The parser will continue as a CSS file is being loaded, although it will block rendering until it has been loaded and parsed (more on that in a bit). When the parser comes across an external resource like a CSS or JavaScript file, it goes off to fetch those files. The first step of this parsing process is to break down the HTML into tokens that represent start tags, end tags, and their contents. The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the web. When the browser begins to receive the HTML data of a page over the network, it immediately sets its parser to work to convert the HTML into a Document Object Model (DOM).
Merge DOM and CSSOM to construct the render tree.The process can be broken down into these main stages: It's from 2011 but many of the fundamentals of how browsers work remain relevant at the time of writing this blog post. I'd highly recommend checking it out.Īlso very helpful was the article How Browsers Work: Behind the scenes of modern web browsers by Paul Irish and Tali Garsiel. This post is a summary of my learnings of the end-to-end process at a fairly high level.Ī lot of this is based on the fantastic (and FREE!) Website Performance Optimization course by Ilya Grigorik and Cameron Pittman on Udacity. My thinking: if I'm going to build websites that are fast and reliable, I need to really understand the mechanics of each step a browser goes through to render a web page, so that each can be considered and optimised during development.