Keep your own offline codepen.
This shell script, a riff on https://github.com/umpox/TinyEditor , lets you prefill TinyEditor's text areas with files from a directory.
html="$(cat $1/html.html)"
css="$(cat $1/css.css)"
js="$(cat $1/js.js)"
cat<<EOT>"$1.html"
<body
oninput="i.srcdoc=h.value+'<style>'+c.value+
'</style><script>'+j.value+'<\/script>'">
<style>
textarea,iframe{width:100%;height:50%}
body{margin:0}
textarea{width:33.33%;font-size:18}
</style>
<textarea placeholder=HTML id=h>${html}</textarea>
<textarea placeholder=CSS id=c>${css}</textarea>
<textarea placeholder=JS id=j>${js}</textarea>
<iframe id=i>
EOT