This is a quick article I wanted to write on how I fixed the black screen issue when exporting my games to HTML5 without switching from GLES3 to GLES2 (which can cause a whole range of performance and other unintended issues).
What worked for me was extremely simple, however, I'm having trouble replicating the initial problem with the newer Godot version. So give this a shot and see if it works for you and if it does/doesn't let me know.
To start things off, this was the only error I was getting when black screen was occuring in HTML5 exports. I still receive this error. This led to just play with the boot splash settings.
What I found is simply by enabling the 'FullSize' and 'Use Filter' options, the game now displayed correctly in HTML5 mode. Weirdly, this also fixed the issue for other games on the same system that had the issue, even when launching them from itch.com. However, when testing those builds on my cell phone, the issue still persisted UNTIL I re-uploaded the new, fixed version.
In summary, if you're experiencing a black screen on your HTML5 exports using the Godot Engine, try enabling 'FullSize' and 'Use Filter' options in your projects settings (project settings -> application -> boot splash). Let me know if this helped you out and check out our youtube page for more Godot tutorial and information about out new and upcoming releases.