Add spritesheet unit test for complying with css requirements

Rename 88m4 to _88m4 to comply with css requirements

code/modules/projectiles/guns/pistols.dm

@@ -586,7 +586,7 @@
 	name = "\improper 88 Mod 4 combat pistol"
 	desc = "Standard issue USCM firearm. Also found in the hands of Weyland-Yutani PMC teams. Fires 9mm armor shredding rounds and is capable of 3-round burst."
 	icon = 'icons/obj/items/weapons/guns/guns_by_faction/uscm.dmi'
-	icon_state = "88m4"
+	icon_state = "_88m4" // to comply with css standards
 	item_state = "88m4"
 	fire_sound = "88m4"
 	firesound_volume = 20

code/modules/unit_tests/spritesheets.dm

@@ -2,10 +2,14 @@
 /datum/unit_test/spritesheets
 
 /datum/unit_test/spritesheets/Run()
+	var/regex/valid_css_class = new(@"^([\l_][\w\-]|[\l_\-][\l_])")
 	for(var/datum/asset/spritesheet/sheet as anything in subtypesof(/datum/asset/spritesheet))
 		if(!initial(sheet.name)) //Ignore abstract types
 			continue
 		sheet = get_asset_datum(sheet)
 		for(var/sprite_name in sheet.sprites)
 			if(!sprite_name)
 				TEST_FAIL("Spritesheet [sheet.type] has a nameless icon state.")
+			if(!valid_css_class.Find(sprite_name))
+				// https://www.w3.org/TR/CSS2/syndata.html#value-def-identifier
+				TEST_FAIL("Spritesheet [sheet.type] has a icon state that doesn't comply with css standards: '[sprite_name]'.")