Microsoft Fluent Design for Web (CSS framework)
Take a look at Microsoft Web Framework for details on the components, HTML structure and CSS that you would use to meet Microsoft's web design principles.
April 2020 Update primary destination for Fluent Design is Microsoft's Fluent Design System website.
For official web frameworks go to github repos:
- Fluent UI - A set of React components for building Microsoft web experiences. Good support and big following.
- Office UI Fabric with only includes grid, typography, icons and utilities. No component and no dependency on React. Not very well maintained.
Some tutorials are available here.