Is the “missing Unicode character” box a Unicode character itself, and if so, which one?

When browsing the web in Windows XP, Windows is often unable to render more esoteric unicode characters. When this occurs, it replaces the characters with a small box.

If you’re running Windows XP, the following should render as this box, for example: ︙

Is this box a Unicode character itself?

If so, I was thinking that I could put it in开发者_运维知识库 my web pages, and see if other Unicode characters I was attempting to use were the same width as it, in order to check whether the other characters were supported or not.

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It is a Unicode code point. It may represent a single character or be one of the combining points (multiple code points forming a character)