HTML comments must start with the four character sequence

1. <
2. !
3. hyphen-minus
4. hyphen-minus
5. <!--

Following this sequence, the comment may have text, with the additional restriction that

1. the text must not start with a single >,
2. nor start with a hyphen-minus followed by a > character,
3. nor contain two consecutive hyphen-minus characters
4. nor end with a hyphen-minus character

Finally, the comment must be ended by the three character sequence

1. hyphen-minus
2. hyphen-minus
3. >
4. -->

<!-- HTML comment -->

/* This is a CSS comment */

// This is a JS comment

/* This is a JS comment */

                       

W3C Comments