Every time I think I couldn't possibly make it smaller, I manage to shave off a few more bytes. Got it under 400 now:

x.arc&&c.parentNode.replaceChild(g=c.cloneNode(),c);x=g.getContext`webgl`;f=Float32Array;eval(`d=320()395(41,318())310(41,new f([0,n=0.5,-n,-n,n,-n]),46)${z='382(a=322(13'}9),'attribute vec2 c;${d='void main(){gl_'}Position=vec4(c${w=",0,1);}')313(a)302(d,a)"}${z}8),'${d}FragColor=vec4(0,1${w}376(d)393(d)435(0,2,128,0,0,0)406(0)404(12,0,3)`.replace(/\d{2,}/g,"\nx[Object.keys(x.__proto__)[$&]]"))