three.js
An open-source 3D library over WebGL with scenes, cameras, materials, model loading, animation, and post-processing.
Why use it
A broad ecosystem and examples suit interactive projects needing a custom web runtime rather than a full game editor.
Where it fits
Find engines, frameworks, code libraries, version control, and production automation.
What to check
Game state, physics, asset pipelines, and performance budgets are yours to design, and upgrades may affect add-ons.