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. 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.
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.
We use the external style sheet method to insert CSS. We define style sheets from another .css file using the <link> tag, and define all of our CSS code in those sheets to stylize the website. We also use the Bootstrap template to structure the website with CSS on a screen and on a mobile device by using the classes row and col.