And besides all that, upper case letters take more ink. Pretty soon your screen is bound to run out of it. Then where will you be?
Seriously, legibility is the issue but legibility is not just a function of u/lcase, it is also a function of the DIFFERENCE between something that you want to emphasize and all else that surrounds it.
As color came onto the scene it represented one more way to make something look different from its surroundings (and made it easy to spot errors like missing closing quotes. Even these days when color is available on almost all the monitors in a workplace, it is not so available on all the printers in that workplace.
U/lcase is sort of a weird one. If you look at a lot of code where all the keywords are in lower case, upper case looks strange and unnecessary. But if you look at a lot of code where all the keywords are in upper case, it seems harder to differentiate lower case keywords from the data.
I've been in environments where common tags and terms that everyone knows are usually written in lower case, but those tags and terms that you don't see from day to day are usually written in upper case.
<td style="text-align: left;" WRAP class="myClass">
I'm not sure why we're here, but I am sure that while
we're here we're supposed to help each other