find

$ find /var/www/html/ -regextype posix-egrep -regex '^.*(css|html)$'
/var/www/html/Bootstrap4/index.html
/var/www/html/Bootstrap4/assets/css
/var/www/html/Bootstrap4/assets/css/bootstrap-flex.css
/var/www/html/Bootstrap4/assets/css/bootstrap.css