Comment by myfonj
You don't even need the base64 encoding for dataURIs: just throw the text payload after mime-type and a comma:
data:text/html,<!doctype html><title>Hi!</title><p>Hello.
Base64 is indeed good to be "safe" and/or somewhat 'conceal' the payload, but it also makes it larger by 1/3 (every three bytes of input become four characters of the base64 output). So taking the risk some devices would not like raw "ASCII dataURI", the QR of the backrooms QR could shave off 738 bytes.
BTW, this is my "HTML sandbox" for testing stuff in a browsers that I summon daily through keyword bookmark to test simple stuff:
data:text/html;charset=utf-8;verbatim,<!doctype html><html style="color-scheme:dark light"><title>HTML sandbox 2.0.6</title><meta name=viewport content=width=device-width,initial-scale=1><body style=margin:0;display:flex;height:100vh onload="OT=(DC=document).title,H=(L=location).hash.slice(1)||'',RX=/(^data:.+?(;verbatim)?,)?([^]*)/,A.value=H.match(RX)[2]?H:decodeURIComponent(H)||A.value;T=W=0;E=RegExp('^'+(D='data:text/html;charset=utf-8,'));F=()=>{if(W!=(V=A.value))W=V,M=V.match(RX),I.src=M[2]?V:(M[1]||D)+encodeURIComponent(M[3]),DC.title=NT=((TM=V.match(/<title\b[^]*?\x3E([^]*?)<\/title\b/m))&&(NT=TM[1])&&(NT=NT.trim())&&(DC.title=NT+' @ '+OT))||OT};F()"><textarea autocapitalize=off style=resize:horizontal;width:50vw autofocus id=A onkeyup=clearTimeout(T);T=setTimeout(F,400) onblur=try{history.pushState({},NT,'\u0023'+(S=I.src.replace(E,'')))}catch(e){L.hash=S}><!doctype html><html lang="en" style="color-scheme: dark light;">%0A<meta name="viewport" content="width=device-width, initial-scale=1">%0A<title>%0A%0A</title>%0A<style>%0A%0A</style>%0A<body>%0A%s%0A<script>%0A%0A</script>%0A</textarea><iframe style=border:0;flex-grow:1;width:0 id=I>
I think I'm going to find this useful.
Duck browser doesn't allow it to be saved as a bookmark, but Safari is fine with it.