HTML is the bit that says what's important. It's good for structuring information. In fact, it's intended to hold semantic information. You can learn about how to use semantic html at that link. It's pretty straightforward!
Learning that HTML is semantic also made me realize what the “headings” in Word and other word processors actually does! It isn't just incredibly ugly but editable formatting presets. This lets it generate a table of contents, either for actual ToC purposes, or for use while outlining your work. This blew my mind; while I moved on to Scrivener for word processing and organization years ago, this was still cool information that I wanted to share! I'm not sure of the full implications of this, nor of the implications of this being or not being common knowledge. Maybe computer classes in school teach this, but I somewhat doubt it. My mom was no help, she was too thunderstruck that I'd left the house without knowing this. The cobbler's children might have shoes, but they definitely don't know how to make them.
CSS specifies how your content looks. That's why HTML has ‘em’ for emphasis and ‘strong’ for “bold.” It's the CSS that makes 'em' look like italics, usually: and that means that in a single place, you can make ALL the 'em' tagged stuff look a specific way, any way at all.
The semantic html also provides useful hooks to attach CSS rules to. But keep the sematic tags useful by using them sparingly! There are also div and span tags to use as hooks for design elements.