This problem has been discussed quite a few times on the forum before, such as:
- HTML projection to object in scene - Demos and projects - Babylon.js (babylonjs.com)
- HTML in 3D Space Part 2 - BJS Way to Matrix Transform - Questions - Babylon.js (babylonjs.com)
- Rendering an iframe on Babylon - Questions & Answers - HTML5 Game Devs Forum
I think the easiest method would be to draw the HTML contents into a canvas using something like rasterizeHTML.js by cburgmer or this Rendering HTML to an image - Ron Valstar - front-end developer, and then using that canvas as the input to a DynamicTexture Dynamic Textures | Babylon.js Documentation (babylonjs.com) which is the texture source of a Rectangle.