A Python script and a pre-commit hook that calls it for validating all the HTML files in a directory (recursively) on commit.

pre-commit

# this goes in your local Git repo at .git/hooks/pre-commit

#!/bin/sh
errors=0
for i in $(find . -type f -name "*.html")
    do
        printf "$i:\n"
        validate $i
        exit_status=$?
        if [ "${exit_status}" -ne 0 ];
            then
                errors=1
        fi
        printf "\n"
    done
if [ "${errors}" -ne 0 ];
    then 
        echo "there were errors, commit aborted"
        exit 1
fi

validate

#!/usr/bin/env python3
"""
This goes somewhere on your PATH, I've got it at /usr/local/bin
Make sure to `chmod +x <this script>` to make it executable!

Validate an HTML file using the W3C HTML validation API
https://github.com/validator/validator/wiki/Service-%C2%BB-Input-%C2%BB-POST-body
"""
import os
import pathlib
from pprint import pprint
import sys

import requests

W3C_VALIDATE_URL_POST = "https://validator.w3.org/nu/?out=json"


def validate_html(args):
    filename = 'index.html'
    if args:
        filename = args[0]
    with open(filename) as fout:
        html = fout.read()

    headers = {'content-type': 'text/html', 'charset': 'utf-8'}
    response = requests.post(W3C_VALIDATE_URL_POST, headers=headers, data=html.encode())

    response_json = response.json()
    if not response_json['messages']:
        print('valid!')
    else:
        pprint(response_json['messages'])
        sys.exit(1)




if __name__ == "__main__":
    if len(sys.argv) > 2:
        raise Exception('please only supply an HTML filename or nothing')
    validate_html(sys.argv[1:])