[int_19h]

Even further back in the day, the Algol-60 reference language used bold and/or underlining for keywords - handy when all you have is a typewriter:

http://www.softwarepreservation.org/projects/ALGOL/report/Al...

http://www.algol60.org/reports/algol60_rr.pdf

(Although technically this was actually syntax, not just syntax highlighting, since that's how the reference language distinguished between keywords and identically spelled identifiers.)