I think most browsers support this feature without it being provided for in the web page. I use a number of browsers and with all of them I just hold down the control key and hit plus or minus to change the font size. This has worked for many years now. Another option is to hold down the control key while using the scroll wheel to make the font bigger or smaller.
Yes it is nice to control the font size and I see on-line news papers using it. Older people often like bigger fonts, but there are many reasons which are valid. Anyway, I think there is no point in having it on web page any more.
DID YOU MEAN THIS?
It just occurred to me that you might be talking about variable size in posting rather than from the client's point of view. In that case there is the heading tag, although there is no range.