To make sure any file or folder you create in /var/www/html gets automatically owned by www-data you can use inotify, it's like cron but monitors folders/files for changes in attribuets, file creations, modifications and much more.

install incron

sudo apt install incron

Allow root to use incron by opening /etc/incron.allow with (and add root to the file, then save and exit):

sudo nano /etc/incron.allow

Edit your incrontab with:

sudo incrontab -u root -e

and add the following line to it:

/var/www/html IN_CREATE /bin/chown -R www-data:www-data /var/www/html/