When a web browser's rendering engine parses an HTML document, it is explicitly programmed to completely ignore anything wrapped inside a comment block. These act as invisible sticky notes attached to your code, helping your future self and your engineering team understand *why* a decision was made.

The specific syntax requires an opening tag consisting of an angle bracket, an exclamation mark, and two dashes (<!--). It must be closed with two dashes and an angle bracket (-->). Mastering this precise syntax is essential, as an unclosed comment can accidentally hide large sections of your website from the end-user.

In massive web applications, an HTML file can contain thousands of lines of code. Professional developers use comments to create clear visual boundaries, explicitly marking where structural sections like headers, main content areas, and footers begin and end.

Beyond textual notes, developers frequently use comments as a powerful debugging tool to temporarily disable specific blocks of code without permanently deleting them. This practice, known as 'commenting out', allows you to isolate rendering issues or safely hide unfinished features from the production view.

There is a critical security paradigm you must understand: while comments are hidden from the visual browser viewport, they are absolutely not private. They are still transmitted over the network as part of the initial HTML payload.

Anyone browsing your website can simply right-click and select 'View Page Source' to read every single comment you have written. Therefore, you must never store sensitive information inside client-side HTML comments. Leaving API keys, passwords, or internal security notes in HTML comments is a catastrophic security vulnerability.