How do I refine the HTML & CSS skills I am learning?

Practice :) html and css are fun to learn. That's what I would do, build something without outside help. You could keep the documentation next to you to get the hang of the syntax but when you're building something without outside help that's when you start learning. Start small, 3 page website with a navigation bar and add some style!

Continual practice and use of the skills. You'll never stop learning. You'll likely never feel as if you are proficient. Just when you think you are starting to have a grasp on it, something will be depreciated, browser support will change, syntax will change, something will or won't need prefixed.

Two big recommendations. Observe the code of some of you favorite sites, and see how they did things. See if you can find HTML syntax issues. See if you can guess what CSS declaration they used before you actually look.

Another thing is to always continue to code. Recreate a website with a new Codepen, and see how close you can get to matching the site without looking at the actual code first. Or if you have project(s) you want to tackle, go for it.

The first step to not suck at something, is to start sucking at something. You'll get there. It's an organic learning process.

If you're stumped with support and CSS prefixes make sure you have caniuse.com bookmarked. That usually has the most up to date information.

Codepen, Codrops, JSFiddle, Github search for "CSS"...

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Never stop learning :)