Baseline (Widescreen) Solution & Issues
Game is Vert- by default.
Go to \SteamApps\common\Half-Life\valve, and open autoexec.cfg. Add this line to it:
default_fov xxx
I recommend 100 for 16:10 monitors and 107 for 16:9 monitors. (default is 90)
Widescreen resolutions are shown in the Video options menu if you set the "Display mode" to "Widescreen."
The game is Hor+ dependent on your FOV setting.
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Same as widescreen but default_fov has to be set to 123 for Hor+.
HUD is small and biased to the left of the screen.
Eyefinity / Surround Specific Solution & Issues
4k UHD Specific Solution & Issues
Same as widescreen. HUD, UI etc. are tiny.