For instance, if a class defines a width of 100 pixels, all DIVS referencing that class will be 100 pixels wide. You can override a DIV's CSS class definition by updating the DIV's HTML code.

Simplest tip ever: Use Visual Studio's Go To Definition with CSS classes to get from your HTML to the relevant CSS rule. First click on the name of a class in your HTML: ...

Google’s Search Relations team explains how CSS affects SEO, addressing myths about class names, pseudo elements, and page performance.