When should one use HTML entities?

Based on the comments I have received, I looked into this a little further. It seems that currently the best practice is to forgo using HTML entities and use the actual UTF-8 character instead. The reasons listed are as follows:

1. UTF-8 encodings are easier to read and edit for those who understand what the character means and know how to type it.
2. UTF-8 encodings are just as unintelligible as HTML entity encodings for those who don't understand them, but they have the advantage of rendering as special characters rather than hard to understand decimal or hex encodings.

As long as your page's encoding is properly set to UTF-8, you should use the actual character instead of an HTML entity. I read several documents about this topic, but the most helpful were:

• UTF-8: The Secret of Character Encoding
• Wikipedia Special Characters Help

From the UTF-8: The Secret of Character Encoding article:

Wikipedia is a great case study for an application that originally used ISO-8859-1 but switched to UTF-8 when it became far too cumbersome to support foreign languages. Bots will now actually go through articles and convert character entities to their corresponding real characters for the sake of user-friendliness and searchability.

That article also gives a nice example involving Chinese encoding. Here is the abbreviated example for the sake of laziness:

UTF-8:

這兩個字是甚麼意思

HTML Entities:

這兩個字是甚麼意思

The UTF-8 and HTML entity encodings are both meaningless to me, but at least the UTF-8 encoding is recognizable as a foreign language, and it will render properly in an edit box. The article goes on to say the following about the HTML entity-encoded version:

Extremely inconvenient for those of us who actually know what character entities are, totally unintelligible to poor users who don't! Even the slightly more user-friendly, "intelligible" character entities like θ will leave users who are uninterested in learning HTML scratching their heads. On the other hand, if they see θ in an edit box, they'll know that it's a special character, and treat it accordingly, even if they don't know how to write that character themselves.

As others have noted, you still have to use HTML entities for reserved XML characters (ampersand, less-than, greater-than).

You don't generally need to use HTML character entities if your editor supports Unicode. Entities can be useful when:

• Your keyboard does not support the character you need to type. For example, many keyboards do not have em-dash or the copyright symbol.
• Your editor does not support Unicode (very common some years ago, but probably not today).
• You want to make it explicit in the source what is happening. For example, the   code is clearer than the corresponding white space character.
• You need to escape HTML special characters like <, &, or ".