Free Software for HTML
GDevelop
GDevelop is a 2D cross-platform, free and open-source game engine that takes visual programming to the next level. GDevelop allows you to take read-made behaviours and add them to your game objects or create new once with intuitive and east to use events. With GDevelop, it is easy to make games for PC and Mobile, as well as launch HTML5 games playable right in your web browser.
Bootstrap
A free and open-source HTML and CSS framework dedicated to responsive and mobile-first frontend web development. With Bootstrap developers can get up and running fast without writing boilerplate HTML and CSS. Bootstrap comes packed with design templates and standards for buttons, forms, navigation, typography and interface components. The framework also provides extra interactive JavaScript extensions written specifically for Bootstrap.
new.css
new.css is a classless CSS framework to write modern websites using only HTML. The framework weighs in at jut under 5kb making it an extremely performant.
PureCSS
PureCSS is a free and open-source CSS framework with a minimal footprint. The entire framework and its modules weighs in at just 3.7KB minified and gzipped. With PureCSS web developers can craft responsive UI for websites that works for mobile as well as desktop. Since every line of the framework was carefully considered, you can rest assured that your page size will be kept to a minimal, and your website loads fast.
Materialize CSS
A modern responsive front-end framework based on Material Design. With Materialize CSS you can speedup your design and development process as most of the heavy lifting is done for you with default stylings that incorporate custom components. All principles of Material Design is incorporated into the framework and can be easily utilized with components and animations that provide better feedback to the users of your websites and applications.
Semantic UI
Semantic UI is a free and open-source framework for HTML and CSS. The framework treats words and classes as exchangeable concepts. Classes use syntax from natural languages like noun/modifier relationships, word order, and plurality to link concepts intuitively. Semantic UI provides similar benefits to that of BEM or SMACSS style guidelines.
Cirrus.CSS
Cirrus.CSS is a component and utility centric SCSS framework designed for rapid prototyping of web sites and web applications. In total, Cirrus.CSS weighs in at 17.3kb GZIPPED and provides all the necessary components for creating forms, buttons, check boxes, inputs and layouts for mobile and desktop.
UIkit
UIkit A lightweight and modular front-end framework for developing fast and powerful web interfaces.
Bulma
Bulma is a free and open source HTML & CSS framework based on Flexbox. With Bulma you can quickly design responsive websites with pre-built components that follows web standards and best practices. The framework provides an easy to understand class structure that makes it easy to design websites with well structured code. Bulma is loved by many, and used by more than 200,000 developers worldwide.
OpenUserCSS
With OpenUserCSS you can change the look of other websites. OpenUserCSS contains a wide range of themes that are easy to apply, to make websites more readable.