Free HTML and CSS templates sites

https://www.free-css.com/free-css-templates

https://templatemo.com/

(has free and premium) https://themewagon.com/

https://html5up.net/

(has free and premium) https://startbootstrap.com/themes

https://www.zerotheme.com/

(has free and premium) https://www.html5xcss3.com/

https://www.w3schools.com/w3css/w3css_templates.asp

https://www.tooplate.com/free-templates 

Note that OceanWP free theme on WordPress.org has a collection of templates which seem to be quite impressive. But the problem seems to be that these templates have to be used only in WordPress environment and cannot be used in an HTML & CSS static website.