Youtube supports some formatting in WebVTT (.vtt) subtitle files. It may only be visible after the subtitles are "published", and viewed on the standard youtube page, not in the edit page.
The advantage of .vtt is that in it's basic form, it's very close to the common .srt format. Just add "WEBVTT" at the top, followed by an empty line, and replace commas in timecodes by dots.
If you are on Mac or Linux, you can convert a SubRip file to .vtt with this command:
perl -pe 'BEGIN {print "WEBVTT\n\n"}; s/(\d),(\d)/$1.$2/g' yoursubs.srt >newsubs.vtt