A rendered HTML preview would be useful

Something like this in `index.html`:
```html
…
{
let p = document.createElement('iframe')
p.id = 'preview'
p.style.cssText = 'width:100%; border:solid grey 1px; padding:0'
p.onload = ()=>{
p.style.height = p.contentDocument.documentElement.offsetHeight + 'px' }
document.querySelector('.container').appendChild(p)
let r = document.querySelector('#results')
new MutationObserver(()=>{
p.srcdoc = r.classList.contains('language-html') ? r.innerText : ''
}).observe(r, {childList:true, subtree:true})
}

…
```