The most annoying part about learning HTML is: you write code but have no idea where you went wrong, spend ages modifying it yet see no results, or click around endlessly trying to find the corresponding element.
Here’s an HTML online preview perfect for beginners: Paste your finished HTML/CSS code into it, and you can preview the effects in real time right away. Click a line in the code, and the corresponding element on the page will be automatically highlighted; click an element on the page, and the matching line in the code will be directly marked. Beginners can tell at a glance which part of the page their code corresponds to, making debugging extremely convenient!