Here are some gui abstractions for ofelia v3. The toggle and the osc abstraction are modified patches from @Cuinjune (thanks for the help). The slider is my creation, I am sure it can be optimized. Any suggestions?
It only has the jump on click behaviour yet, I did not manage to implement the steady on click behaviour.
The abstractions work with creation arguments.
Toggle: xPos, yPos, size.
Slider: xPos, yPos, width, height, minVal, maxVal.
Osc: xPos, yPos, width, height.
Lis: xPos, yPos, size.
I will try to add some more abstractions in the future. Not as easy as expected, but possible
Would be nice to have a collection of GUI elements for ofelia.

ofelia3_toggle_slider_osc_lis_abstractions.zip

And here is something completly different, a port from the shaderExample from openFrameworks:
ofeliaGlShaderExample.zip

I tested emscripten with this patch. I had to install Linux, because i could not make it work with Windows. Is it possible with Windows at all? I also had to install a lot of dependencies, but finally it works.
Here is the test result: http://puredata.handmadeproductions.de/
Somehow i can not compile the patch with the oscillater and lissajjous abstractions, so i deleted them for the Emscripten test. Everything else works fine (audio with Firefox sounds good but not with Chrome - i only tested those 2 browsers).
@Cuinjune Do you know why those abstractions do not work? Is it because of pdArray? I already deleted [switch] inside the abstractions because i read in ofApp.h: const int ticksPerBuffer = 8; // used to compute the audio buffer len: tpb * blocksize (always 64). I assume that means there is only one blocksize possible?
Edit: I think it is because [bang~] updates the arrays too fast, it does work with [metro 100].
Another issue is that the font is bigger than expected (it is basically the patch from above).

Very nice way to put Pure Data patches to the web

Another question:
I tried to compile a patch that outputs midi notes, but no luck so far.
Is it possible to input and output MIDI data to and from the browser with Emscripten?

There is also this message when I run the patch:

MidiInDummy: This class provides no functionality.
MidiOutDummy: This class provides no functionality.

I dont know if it is possible at all, but this sounds quite promising:
https://www.midi.org/17-the-mma/99-web-midi

the oscillator and lissajous abstraction do work now with emscripten. the two object that do not work (at least for me at the moment) are [bang~] and [midiout], nothing works if they are part of the patch. [noteout] gets compiled, but i dont receive anything. and the issue with the fontsize and the linewidth and (perhaps) antialiasing of the lines...
would be great to have a small ([midiin] and) [midiout] example, if it is possible.

@Jona Hi man, sorry for the late reply and thanks for sharing your GUI work and example you ported.

I'm glad you found out the way to make your abstractions work.

However, in my chrome browser, the audio was glitchy so I changed const int ticksPerBuffer = 8; to const int ticksPerBuffer = 32; and now it seems to work fine. I also updated this in Ofelia Github repo: https://github.com/cuinjune/Ofelia/commit/b4b0b11872574eb1046e3b15cc9011982559ad42

Could you please update the ofelia3_toggle_slider_osc_lis_abstractions.zip patch in your original post and also, rebuild the project using const int ticksPerBuffer = 32; and update files in http://puredata.handmadeproductions.de/?

I'm not sure if I can make MIDI work in the browser or not. I will have to try different things or ask people around.

@cuinjune thanks for small update. I had distorted audio with firefox too. Now it sounds "good" / like expected.
On Android it is possible to use the GUI with the touchscreen...
I reuploaded ofelia3_toggle_slider_osc_lis_abstractions.zip in my original post (it is now exactly what I used for the Emscripten port) and http://puredata.handmadeproductions.de/

Another two questions: Is it possible to play/embed video/audio files with Emscripten?
I tried to port the video player patch with an embedded video file and I get this message when i run "emrun":

exception thrown: RuntimeError: float unrepresentable in integer range,RuntimeError: float unrepresentable in integer range
    at wasm-function[3404]:0x114faf
    at wasm-function[5250]:0x1b5b7b
    at wasm-function[593]:0x288ff
    at wasm-function[343]:0x13515
    at wasm-function[2019]:0x9bb5f
    at wasm-function[1743]:0x8864d
    at wasm-function[593]:0x288ff
    at wasm-function[4502]:0x176e26
    at wasm-function[593]:0x288ff
    at wasm-function[343]:0x13515

There was also a message with "broken pipe".

My second question is if shaders work with Emscripten and Ofelia. I assume if then only webGL ES shaders? I ported the displacement map example to Emscripten, it shows the map but it seems that the shader takes no effect.
Edit: I just read the comment in the ES2 shader.vert file:

// PLEASE NOTE.
// texture2D() is not supported in vertex ES2 shaders.
// so unfortunately on ES2, we can not sample the texture,
// and displace the mesh.
// Please see GL2 and GL3 examples for displacement mapping.

Is texture2D() supported in fragment ES shaders?

video and audio files are possible with emscripten. the error message above was because i was returning a float with too many digets (the video position).

it should go away with the build flag -s "BINARYEN_TRAP_MODE='clamp'" but somehow the flags seem not to work for me.

the same with flags for increasing memoy: ALLOW_MEMORY_GROWTH = 1, but also TOTAL_MEMORY = 16777216, ABORTING_MALLOC = 1. In addition I tried WASM = 1 with WASM_MEM_MAX.
here are all build flags: https://github.com/emscripten-core/emscripten/blob/master/src/settings.js

right now I have a frame buffer with 50 frames, with more than 78 frames it runs out of memory (OOM).
so here is my second test, the slitscan patch: https://slitscan.handmadeproductions.de/
main.pd
this time the patch works better with firefox than with chrome.
still it slows down a little over time, that´s something i would like to fix.

the website says it is not a save connection, but i can guarantee it is don´t know how to avoid that warning.