What Is a Markup Language? HTML is used to mark up the structure of Web pages, but other markup languages are used by computers on the Web and beyond. The Standard Generalized Markup Language (SGML) is the grandfather of most markup languages used for print and the Internet. SGML is the international standard used to define the structures and appearance of documents. Multiple SGMLs have been created for a variety of document types and for various specialties—such as physics, accounting, and chemistry. HTML and XHTML are the Web’s primary version of SGML.

It’s sometimes difficult to keep track of all the values that you are using in your design. It’s unlikely that CSS will ever include constants, so it will help to keep notes in an easy-to-reference location in your document. Creating a glossary of colors and types leads to more consistent and attractive designs  Although section headers and dividers really aren’t anything more than CSS comments, they do help organize your CSS and allow you to quickly scan your code to locate particular CSS rule groups. If you have established a TOC, I recommend reflecting that organization here.