I’ve been looking at XHTML specifications recently and was surprised at the number of special symbols that are defined.
From simple things like ¬ ¬ and © © — to much more esoteric ones like ƒ ƒ. I can see the need for them, but the sheer number means that unless you use them all the time there will be the need to look them up.
I like the math set though, ∀ ∀ ∂ ∂ ∃ ∃ ∅ ∅ ∇ ∇ even if I have never had much use for these outside of math and logic classes. Similarly the arrows are nice ↔ ↔ looks neat, but not something I use a lot.
Overall there are some useful symbols in XHTML, and it sure beats trying to remember the key code to type them. After all ¦ ¦ is easier to type than trying to find the broken pipe symbol on the keyboard. § § and ¯ ¯ could also be useful when annotating documents.