How do I create a search that returns multiple file extensions?

A Spotlight search will do this for you. Assumption that you have Search Results selected for Documents, Developer, and maybe Other in Spotlight preferences.

You can enter this in a regular Finder Window's Search field, or in Spotlight. At the bottom of the results for Spotlight, select Show All in Finder.

If you save the following as an AppleScript script, or script bundle, you can double-click on it to produce a new Finder window with the Spotlight search results discussed previously. There is a bug in AppleScript that prevents it from setting the search results view as Arrange by Name, so you will need to do that manually.

Copy/paste into Script Editor and save accordingly.

-- Applies a Spotlight search to open a new Finder window with HTML, CSS Style Sheets,
-- and Javascript files shown in icon view. Need to manually adjust Finder's Arrange by
-- kind to Arrange by Name for uninterrupted icons.

-- The Arrange by Name directive in the code is ignored by Finder

use framework "Cocoa"
use AppleScript version "2.4" -- Yosemite or later
use scripting additions

property NSWorkspace : a reference to current application's NSWorkspace

set query to "(kind:HTML OR kind:Style OR kind:JavaScript) AND (name:.html OR name:.css OR name:.js -(name:.json))"
set result_code to (current application's NSWorkspace's sharedWorkspace)'s showSearchResultsForQueryString:query
if result_code = 0 then return

tell application "Finder"
	tell front Finder window
		set its current view to icon view
		set its arrangement of icon view options to arranged by name
		set its text size of icon view options to 14
	end tell
end tell
return

This will suppress .json files.

name:.html OR name:.css OR name:.js -(name:.json)