CSS (acronym for Cascading Style Sheet) is a style sheet language. Whereas HTML structures and displays content on the web pages, CSS stylizes the presentation, display, and layout of the web page on different devices and screen sizes.
There are three ways to insert CSS into an HTML document: External style sheet, inline style sheet, and inline styling. External style sheet defines style sheets from another .css file and includes it using the HTML tag <link>. You can implement your CSS code in the .css file and change the look of your website from a file. Defining a tag in the .css file will modify all of the same tags in the HTML document. Inline style sheet is defining style sheet rules within the <head> tags in the HTML document by creating the <style> tag. Inline styling applies direct CSS code within HTML tags using style.
Out of all of the ways to insert CSS, external style sheet is considered the best practice because it aligns with the idea of separating structure from style, while inline styling is considered worst because of the hectic method of changing each tag one by one when you can change all of them in one snippet of code in external and internal style sheets.
CSS3 is the current version of CSS and introduces new features to further stylize the look of your page. These features include new selectors, box model, backgrounds and borders, animations, etc. Browsers that support CSS3 include IE, Firefox, Chrome, Safari, and Opera.
CSS3 is the style sheet language we use to design the presentation, display, and layout of the web page created by HTML. It's useful for us for scaling HTML content on different devices and screen sizes after we have fully creating a page for an application.