What's HTML character code 8203?

It's the Unicode Character 'ZERO WIDTH SPACE' (U+200B).

this character is intended for line break control; it has no width, but its presence between two characters does not prevent increased letter spacing in justification

As per the given code sample, the entity is entirely superfluous in this context. It must be inserted by some accident, most likely by a buggy editor trying to do smart things with whitespace or highlighting, or an enduser using a keyboard language wherein this character is natively been used, such as Arabic.

If you want to search for these invisible characters in your editor and make them visible, you can use a Regular Expression searching for non-ascii characters. Try searching for [^\x00-\x7F]. Tested in IntelliJ IDEA.

I landed here with the same issue, then figured it out on my own. This weird character was appearing with my HTML.

The issue is most likely your code editor. I use Espresso and sometimes run into issues like this.

To fix it, simply highlight the affected code, then go to the menu and click "convert to numeric entities". You'll see the numeric value of this character appear; simply delete it and it's gone forever.