How do I get superscript and subscript text in alt attribute? Do I use entities?
The text is about chemical formulas.
Well, you won't be able to do it with markup tags since ALT is plain text. Thus using the Unicode http://en.wikipedia.org/wiki/Unicode_subscripts_and_superscripts or similar as a fake would make most sense though obviously the results will depend on the font and encoding, etc.
If your image is a water molicule, just do "h 2 o" versus h <sub>2</sub> O
Way more info, but an interesting read: http://en.wikipedia.org/wiki/Wikipedia_talk:Manual_of_Style/Accessibility/Archive_8
I'm using utf-8 so I don't think there will be problems with unicode entities, I'll go that way.