The read-only XRWebGLLayer property framebufferHeight indicates the height of the framebuffer, in pixels.

You can get the width of the framebuffer using the framebufferWidth property.

The height in pixels of the XR device's framebuffer. Each of the framebuffer's attachments (pixel, depth, color, and/or stencil buffers, for example) are all this many pixels tall.

This snippet fetches the framebuffer's width and height for later use.

let glLayer = xrSession.renderState.baseLayer;
gl.bindFramebuffer(gl.FRAMEBUFFER, glLayer.framebuffer);

frameWidth = glLayer.framebufferHeight;
frameHeight = glLayer.framebufferHeight;

• WebXR Device API