There was a WebGL extension proposal to support using iframe as a texture, but it was not implemented and it doesn’t seem that any progress is made on this extensions: WebGL WEBGL_texture_source_iframe Extension Proposed Specification
What @Vladan found is a trick to apply 3D CSS transforms to show a web content in 3D. But such content is not really a part of a 3D scene - it is still rendered separately, the same way the current flat HTML Label extension in Shapespark is rendered. So you couldn’t for example map such content as a texture on any object. And unfortunately such content still wouldn’t be visible in VR, the same way HTML Labels are not visible in VR.