Top personal issue, be nice if it could just attach to an <input> and list the result. Like:

  <input type="range" id="example_id" name="example_nm" min="0" max="50">
  <output name="example_result" for="example_id"></output>

And it would just show you the input value. Maybe with a "type" specifier like talked about. Maybe the ::before or ::after css and it would allow content: updates or something.

Bunch of <input> types that there's a reasonable case for. Especially if it allowed for formatting. Did you put in the type="tel" the way you believed? It prints it out formatted.

'checkbox, color, date, datetime-local, file, month, number, radio, range, tel, time, url, week' might all have possible uses. Some of the text cases might have uses in specific conditions. 'email, text, url'

Also be nice if the for="" attribute actually did very much. The attachment seems mostly irrelevant in the examples seen. Most example just use a variation on:

  <output name="result">
  <form oninput="result.value=...">