UI Widget & Component Toolkits

On Web Platform: [^1]
React Specific, On Web Platform:
Native Desktop/Laptop/Netbook Apps via Web Platform (i.e. used with NW.js and Electron):

ADVICE:

[^1] If you need a basic set of UI Widgets/Components start with Semantic UI. If you are building something that needs a grid, spreadsheet, or pivot grid you’ll have to look at Kendo UI or Webix. Keep in mind that most of these solutions still require jQuery.