Painting an unstyled page, and then repainting it once styles are parsed would be bad user experience. For this reason, CSS is render blocking, unless the browser knows the CSS is not currently needed. The browser can paint the page once it has downloaded the CSS and built the CSS object model. Browsers follow a specific rendering path: paint only occurs after layout, which occurs after the render tree is created, which in turn requires both the DOM and the CSSOM trees.
CSS is a language that describes the style of an HTML document. CSS describes how HTML elements should be displayed. CSS stands for Cascading Style Sheets. CSS saves a lot of work becuase it can control the layout of multiple web pages all at once. CSS rule-set consists of a selector and a declaration block , see the following image. The declaration block contains one or more declarations separated by semicolons.