You can use white space to make you code easier to read.
When the browser comes across multiple consecutive spaces, it only reads one space. This goes for line breaks as well, as it treats it like a single space (line breaks meaning hitting enter on the keyboard, not the actual code for a break). This is known as white space collapsing and this indenting can make your code easier to follow
Semantic markup can add extra information to your pages.
Structural markup refers to heading and paragraph elements, but semantic markup refers to extra info such as emphasis and block quotes. The purpose of these is not to necessarily change the way your text looks, although they do, but to describe the content of your web pages more accurately. For example, a voice of a screen reader may change to add emphasis to a word. There are a wide range of options ranging from quotations and citations to changes in content.
Color can bring your pages to life.
You can specify color through color names, hex codes, and RGB values. You can specify opacity as it's own entry or as the fourth listing of rgba in decimals that refer to percentage. Remember that color is an important aspect that can convey mood and evoke reactions. Keep contrast in mind and always think about readability.