I'd actually like to respond with a bit more detail.
-Borderless Fullscreen-Windowed and just regular 'Windowed' modes are crucial to have. If there is only exclusive full screen as an option then it can't be streamed easily without hack-y workarounds.
-There should be resolution support up to and including any monitor resolutions supported by the OS/GPU/Display. There is no excuse. Anything less is laziness because most (decent) game engines come with that level of support right out of the box.
-If applicable, there should be the following options:
Text customization such as font, size, style (bold/italic/etc), color
An in-game gui method to alter the FOV for any 3D games
Colorblind options for visual elements
Text alternatives for audio elements
Cross-platform support for Win/Mac/Linux-based OSes and possibly mobile devices if applicable
-Browser games typically don't command the type of audience that also watches these YouTubers/Streamers (aka enthusiast/hardcore audience). You'd honestly be better off putting browser games cross-platform on mobile as well and making money through passive (sensible) adverts; avoid the rapidly-flashing crap please.
-Try to stream the game yourself with OBS (Open Broadcast Studio) and see how well it works. Having 'streamer friendly' instructions for OBS ready-to-go will also be a nice way for new streamers to jump in and try it out. ^_^