You can’t get very far building the web without an understanding of HTML. HTML defines the structure of web content, and it’s worth your time to familiarize yourself with various elements and their attributes.
Cascading Stylesheets (CSS) define the presentation of web content. Even if you have little interest in becoming a professional designer, you can gain a lot from learning the basics of CSS. For example, you’ll use CSS selector sytax to target elements in JavaScript.
If you already have a decent handle on these topics, you’ll fly right through the lessons, but you may pick up a little bit of new knowledge along the way. I often find that when I review the basics, there’s something that I haven’t thought about in a long time that I ought to be using more often.
One of these courses or the other should be plenty of preparation. Don’t feel like you need to complete both, unless you just want the extra practice.
Codecademy’s free course Learn HTML & CSS: Part 1 has been rewritten from scratch to use the latest versions of HTML and CSS, replacing a much older course. You can complete the course right in the browser. This is a free course. You can ignore the big $199 banner; that’s their attempt at an upsell. (Can’t blame them for trying.)
Dash, from General Assembly, offers several basic interactive courses that teach the fundamentals of modern HTML and CSS—and even a little bit of JavaScript. And it’s all free, browser-based, and based on the latest standards.