I haven't spent much time on this but these notes may save you some time although it is likely that there are better solutions. They are Internet Explorer specific and untested on any other browser. If anyone really knows what we should be doing, please email me at Aidan.Schofield@bristol.ac.uk
It turns out that many mathematics symbols are actually defined in HTML now and so one can make fairly good Web pages with mathematics in them. To see the range of things easily available and how to include them you could look at Alan Wood's Unicode Resources .
In order to get the greatest array of characters available, it seems that the opening HTML tag should read <html lang="en">. Presumably it is only important to specify the language but it had better be the code for a genuine language because I tried <html lang="orange"> and it didn't work. For reasons that I do not understand, on my work computer but not on my home computer, the font Lucinda Sans Unicode does display the symbols ∀ and ⇒ (amongst others) once you have done this but will not if you have not.
Actually seeing the correct things in your browser is also a problem. There are various fonts that claim to be fairly Unicode compatible but most of them do not display all of the characters you may come across in Web pages or need to include yourself. From my cursory investigation of this I found that Lucida Sans Unicode did the best job by far with mathematics. So under Tools -> Internet Options, select Fonts and find this font in the lefthand pane. If you don't find it there, I'm not sure what to do as I could find no way of downloading it after a short look on the Web. If you do have Lucida Sans Unicode and this procedure didn't work then once you have got to Fonts as before then click on the top tab and select "User-defined" under "language script" and assign it to be some useful font; on my home computer Lucida Sans appeared to contain many useful mathematics symbols that were not in Lucida Sans Unicode.