Comment 24 for bug 239854

It would be nice to see mark and underline treated as separate formats.
Currently the usual wiki code for underline is used for mark, which displays as yellow background highlight.
Similar to earlier suggestions, you could introduce a separate code for mark.
But I would suggest an option "treat underline as mark", initially on by default. Those users that wish to use underline would turn this off.
You could also/alternately include an option to "convert mark to new internal format".
In any case I would use underline in place of yellow highlight most of the time.
But the highlight, being highly visible, is very useful for certain exceptional cases.

IMHO, requiring links to have a uniform format is not only good from the programming side, it also is visually more coherant.