In UNIX, thankfully, the key combinations for entering these symbols are mnemonic, and only occasionally fall into the realm of the arbitrary. The compose key is used followed by two other characters (sometimes shifted, etc.) to enter one of these symbols, and the character combinations are things like A and ' for Á, 1 and 2 for ½, etc.

Also note that the numeric HTML codes literally mean "use character number 248" or whatever; these are dependent on the encoding used by the font used to display that text, and may change from one OS to another, or even one font to another within the same OS. Several of the characters above do not match their descriptions in the browser I am using now; for instance, "trademark" is an empty square, and "lessequal" is a 3/4. To ensure your HTML looks correct to everybody, stick with the named HTML entities as listed in E2 HTML Tags.