Just as with a natural language, you have an active and a passive vocabulary with a development language. That active vocabulary is a collection of keywords which you use daily, and understand fluently. The passive vocabulary is a set of keywords which you recognize, but may not think of immediately.
For HTML, you should have an active vocabulary of about 20 - 30 elements. Look over web pages on just about any web site (including Quora) and you'll find the same 30ish HTML elements; <body>, <div>, <p>, <ul>, etc. These you should know well.
Your active vocabulary are those HTML tags that you've read about - from a book or a blog. It's ok that you don't know the half-a-dozen different elements that exist for a <table>. but if you see a <colgroup>, it should seem familiar to you.