Text utilities work on lines (text lines being (not too long) sequences of non-NUL characters terminated by a newline character).
awk is the one standard utility that can be told to work on record separated by other things than newline characters, that's why
awk talks of records instead of lines.
For instance, you could use
> as the record separator. As in:
awk -v RS='>' ...
Another approach is to swap the character you want to use as the record separator (for other tools than
awk) with the newline character:
tr '\n>' '>\n' |
sed ... |
tr '\n>' '>\n'
Those assume that the things you want to modify don't include nested HTML tags as they would start new records.
That is replace