Nice, the borders look great! if you can find out a way to scale the whole window down when the width shrinks below the 16:9 ratio, it should also work well for older style monitors (e.g. 4:3) or tall browser windows too!
You are showing no mercy with the scaling ;-) But tbh, me not either - prepared the border already in a way, that it can also be applied to the portrait scaling. Being just a little scared to start on this one. The expected responsive behavior is pretty obvious, but under the hood a lot of bolts and nuts have to fit perfectly to scale and position everything into the right place.
Totally understand! I saw on your website that you were using a (custom?) non-canvas web stack for this project, and being experienced with professional web dev myself I get how that could be a real challenge. I'm using the Godot engine for my projects which thankfully has many built-in options for handling window scaling, so it's something I occasionally take for granted.
(Btw I can highly recommend Godot as a general purpose game engine, it has some pretty good platform export options, although Godot 4's default web export has a WebGL 2.0 dependency which can cause issues with some browsers, mainly on Macs. It's free & open source, has a great, growing community, and there are a lot of great learning resources available! Maybe something to check out for future projects if that is of interest to you.)