HTML was never intended to describe the subtle details of markup, like color and fonts, and sizes. It was supposed to be used to describe the parts of a document and leave the formatting details to the browser or to a style sheet. However, the various browser vendors invented their own flavors of HTML to do these things, and these elements and attributes became standardized through general usage.
In a word processing package, you might use a pull-down menu to enable the properties of various document elements. Once these properties are setup, you just apply them to new elements as you type, or adjust them as needed.
Similarly, a style sheet describes a property list for an HTML element. You can describe the properties for all occurrences of an element (such as all headings), or you can make special cases for some elements and groups of elements.
There are many ways to specify a style sheet. Let's start by describing the properties in the document HEAD section, then later, go over the other ways.