This comes from the original HTML 3.2 spec (1996) which defined 16 colors matching the Windows VGA palette, where pure #00FF00 was deemed too bright for general

This comes from the original HTML 3.2 spec (1996) which defined 16 colors matching the Windows VGA palette, where pure #00FF00 was deemed too bright for general use, so they chose the more muted #008000 for "green" and later kept it for backward compatibility.