We find them for you so that you don’t have to.
You should note that, even though there may be hundreds of HTML5/CSS3 templates out there in cyberspace, they will almost all be variations around a small number of component: HTML5 pages are effectively ‘Lego’-like. So it’s unlikely that you’ll need to look through too many until you find one that fits your needs.
HTML5 / CSS templates
HTML5 UP
High quality 100% free templates, licensed under the Creative Commons Attribution license
» http://html5up.net
HTML5xCSS3
A huge repository of more than 500 HTML5 / CSS3 templates (as well as numerous tutorials, tools, and other CSS3 resources)
» http://www.html5xcss3.com
StartBootstrap
» http://startbootstrap.com
Theme Wagon
Some really rather interesting and unusual templates among the 64 offered here (my personal favourite is Boots4)
» https://themewagon.com/themes/
FreeHTML5templates.com
Over 100 free templates, several of which are quite unusual
» http://www.freehtml5templates.com
Fresh Design Web: ‘250+ Free Responsive HTML5 CSS3 Website Templates’
» https://www.freshdesignweb.com/free-html5-css3-templates/
W3layouts: 100 Free Responsive HTML5 CSS3 Website Templates
» https://w3layouts.com/free-responsive-html5-css3-website-templates/
Templated
“… a collection of 849 free, Creative Commons-licensed CSS, HTML5, and responsive site templates”
» http://templated.co
One Page Love: 20 free one-page templates
» https://onepagelove.com/templates/free-templates
Webflow
16 free templates
» https://webflow.com/templates
Zerotheme
Some 50+ free themes
» http://www.zerotheme.com/category/free-responsive-html5-themes
DesignBootStrap
Free templates, documentation, and tutorials
» http://www.designbootstrap.com
DesignsCrazed: ’47 Free Responsive HTML5 CSS3 Website Templates’
» http://designscrazed.org/free-responsive-html5-css3-templates/
PixelHunt
» http://pixelhint.com/category/freebies/
PixelsByRick
Two free themes
» http://pixelsbyrick.com/themes.html
WordPress themes
Free Design Web
A collection of collections (by topic) of WordPress themes, most free
» https://www.freshdesignweb.com/category/wordpress-themes/
DesignsCrazed:’30 Free Premium WordPress Themes 2015′
Some excellent themes–definitely ‘premium’ quality
» http://designscrazed.org/free-premium-wordpress-themes/
Tools
Viewport Resizer
Viewport resizer is a browser-based tool to test any website’s responsiveness. Just save the bookmarklet, go to the page you want to test, click on your created bookmarklet and check all kinds of screen resolutions of the page.
The smartest way to share your defined environment of devices and breakpoints directly with your team and client.
» http://lab.maltewassermann.com/viewport-resizer/
Responsive Design Simulator
The responsive design simulator serves one purpose: to help designers, and audiences, visualize their responsive sites on popular devices. Simply type in the URL for your site and view the scrollable simulations below. Keep in mind, this is not a replacement for actual mobile testing as the mobile browsers can render CSS and HTML uniquely.
» http://responsivedesignsimulator.com
MobileTest.me
Test your websites in smartphone & tablet emulators
» http://mobiletest.me