Well, that -- SQL keywords in uppercase -- does tend to highlight the table & column names in lowercase excellently. When everything is in lowercase, a query requires too much effort to pick out its purpose.
Needless to say all uppercase is even harder to parse; doable but I would rather do 0,$s/./\L&/g if only to make reading easier.
Back in the olden days when the world hadn't yet got the hots for the case-sensitive XHTML, I used to ALLCAPS the HTML tags (similar to how the examples in the W3C docs had been written) and had the attributes in CamelCase. Values, where case didn't matter, lowercase. It was a nice distinction and made it easy to read things even without source code highlighting.
I personally prefer CamelCase. No reasons whatsoever but during my undergraduate, one of my faculty used to crib a lot about not using the CamelCase. He'd ask us reprint all the lab-records if we hadn't used CamelCase... so it became a habit and it even makes sense to me... its easier to read.
Well, only one of these should make it through, right?
<COMMENT>UPPERCASE IS BEST</COMMENT>
<comment>lowercase is best</comment>
<Comment>CamelCase IsBest </Comment>
<cOmmEnt>vOwElcAsE Is bEst</cOmmEnt>
<CoMMeNT>CoNSoNaNTCaSe iS BeST</CoMMeNT>
<cOmMeNt>rAndoMCaSe iS BeSt</CoMmEnT>
<for Mayor>None of the above</for Mayor>
For me, tags represent a mood. Proper mood requires uppercase, perl mood requires lowercase, pretty mood required Camel Case. That's got to be it, because i use lowercase in my unseen tags here. Yet on other sites, i may use uppercase, even though it makes no difference whatsoever.
Remember PETSCII? On Commodore computers (C64, C128), the SHIFT+COMMODORE key combination switches the codepage between uppercase and lowercase letters. Perversely, shifted means lowercase. But with this kind of a feature, it doesn't matter if you type your tags in uppercase – just SHIFT+COMMODORE to change them all to lowercase.