Sourced from [webglstats.com].

WebGL 1

WEBGL_extension_pack1_1

blend_minmax: 88%/96% (99% w/o ie+edge)
debug_renderer_info: 100%/100%
element_index_uint: 100%/93% (88% Chrome Android)
instance_arrays: 99%/92% (86% Chrome Android)
lose_context: 100%/100% w/o ie+edge
standard_derivatives: 100%/100%
vertex_array_object: 100%/100% w/o ie+edge

Maybes

depth_texture: 90%/84%
- 96% w/o ie
- 71% android
- 100% iOS
draw_buffer: 67%/1%
- Chrome Windows/macOS/Linux: 71%/95%/47%
- Chromium Linux: 79%
- Firefox Windows/macOS/Linux: 92%/99%/97%
- Exists on Firefox Android (percentage unknown)
frag_depth: 87%/0%
- 92% w/o ie
- 100% Firefox Desktop
- 89% Chrome Desktop?
- 0% both Chrome Android and Safari iOS
- Exists on Firefox Android (percentage unknown)
texture_filter_anisotropic: 99%/74% (only 52% Chrome Android)
texture_float: 99%/75%
- 53% Chrome Android
texture_float_linear: 96%/60%
- 26% Chrome Android
texture_half_float: 93%/74%
- 99% w/o ie
- 53% Chrome Android
texture_half_float_linear: 92%/73%
- 98% w/o ie
- 99% w/o ie+edge
- 51% Chrome Android

WebGL 2

WEBGL_extension_pack2_1?

Likely not enough common extensions to be worth it.

color_buffer_float: 100%/84%
- 84% Chrome Android
disjoint_timer_query: 92%/86%
texture_filter_anisotropic: 99%/45%
- 45% Chrome Android (?)
texture_float_linear: 100%/40%
- 40% Chrome Android (?)