%s/<[^>]*>/\L&/g
Parse:
& is the "memory" of what matched in the LHS. Everything that was matched on
the left side of /.
:%s # substitute over whole file
/<[^>]*>/ # match from < to FIRST > (note use of non-greedy [^>]* which means
# match all characters other then >
\L&/ # Put everything (&) matched into lowercase (\L)
g # Do as many times as possible on line
However, this should be used with care, since it will turn
<img src="Images/PICT01234.JPG">
into
<img src="images/pict01234.jpg">