The answer from @Mr.Alien probably addresses what you really wanted to know. I will answer the question that you actually asked:

There is no HTML entity for a character resembling your image. This does not mean that such characters cannot be used; just the absence of defined entities.

The classic HTML concept of entity reference is being replaced by the “named character reference” in HTML5. Interpreted as relating to them, the answer depends on whether your image is interpreted as “” U+25C0 BLACK LEFT-POINTING TRIANGLE or as “◂” U+25C2 BLACK LEFT-POINTING SMALL TRIANGLE. They are very similar, typically identical in shape but in different sizes. The former has a named character reference, ◂, in HTML5.

However, using the such references (beyond those defined in HTML 4) is unwise: they have limited browser coverage, and they provide no real benefit over using the numeric character references ◀ and ◂ (or their decimal counterparts) or the actual characters.

Note that these characters are present in some fonts only. This problem does not depend on the way the characters are entered in a document. For more on this, see my Guide to using special characters in HTML. (This is where CSS becomes relevant. CSS is totally irrelevant to the basic question about including a character in HTML.)