Quick proof-of-concept add-on for Anki web view hooks PR. Please copy into subfolder in addons21 directory to test.

__init__.py

# Quick proof-of-concept add-on for Anki PR: 

from anki.hooks import addHook

def addCSS(head, instance):
    print(head, instance)
    return head + "<style>.addition1{color:green;}</style>"

def addCSS2(head, instance):
    print(head, instance)
    return head + "<style>.addition2{color:blue}</style>"

def addHTML(html, instance, screen, part):
    print(html, instance)
    return html + "<div class=addition1><b>first {} to {}</b></div>".format(part, screen)

def addHTML2(html, instance, screen, part):
    print(html, instance)
    return html + "<div class=addition2><b>second {} to {}</b></div>".format(part, screen)

# general usage: addHook("deckbrowserWebPrefix", addHTML)

# Heads, prefixes, and suffixes
for screen in ("overview", "deckbrowser",  "bottombar", "reviewer",
               "reviewerBottom", "preview", "editor", "stats", "toolbar",
               "cardlayout"):
    addHook(screen + "WebPrefix", lambda h, c, s=screen, p="prefix":addHTML(h, c, s, p))
    addHook(screen + "WebPrefix", lambda h, c, s=screen, p="prefix":addHTML2(h, c, s, p))
    addHook(screen + "WebSuffix", lambda h, c, s=screen, p="suffix":addHTML(h, c, s, p))
    addHook(screen + "WebSuffix", lambda h, c, s=screen, p="suffix":addHTML2(h, c, s, p))
    addHook(screen + "WebHead", addCSS)
    addHook(screen + "WebHead", addCSS2)

# FIXME?: Toolbar is drawn in mw.setupMainWindow(), i.e. before mw.setupAddons(),
#        thus needs to be redrawn for changes to appear

from aqt import mw
mw.toolbar.draw()

# Stats area in Overview and DeckBrowser

addHook("deckBrowserWebStats", lambda h, c, s="deckbrowser", p="stats":addHTML(h, c, s, p))
addHook("deckBrowserWebStats", lambda h, c, s="deckbrowser", p="stats":addHTML2(h, c, s, p))
addHook("overviewWebStats", lambda h, c, s="overview", p="stats":addHTML(h, c, s, p))
addHook("overviewWebStats", lambda h, c, s="overview", p="stats":addHTML2(h, c, s, p))