What is the A-Frame framework all about? A-Frame is a framework for building Virtual Reality applications using web technology, originally by Mozilla and started in 2015. It is built on top of the WebVR API and uses Three.js. Three.js is built on top of WebGL. This may seem like a dependency on top of a dependency. But, although it is, you probably do not want to write everything for WebGL and WebVR yourself.
A-Frame uses a declarative syntax in your HTML files, which makes your HTML files feel very natural, easy to understand and it makes copy-pasting very simple. A-Frame is, just as the web itself, cross-platform. Applications build with A-Frame run everywhere from your desktop browser to your mobile devices, on Google Cardboard, Google Daydream, and Samsung Gear, and on the HTC Vive and Oculus Rift. It supports the controllers for the various devices as well.
A-Frame is using Three.js in the back, but you are free to use whatever you want next to it on your website. Because it used the HTML component structure you can use whatever you want next to it, like Vue, Angular or React. It also comes with a visual inspector, just hit CRTL+ALT+I when running.
A-Frame uses component you can build yourself or download from a large repository. This list is curated and can be compared with the Unity Asset store a bit. These components make it very easy to extend your application and create reusable pieces.