Fun fact: for servers that are not too particular about the Content-Type they receive or the presence of extra object attributes, it is also possible to submit (fixed) well-formed JSON using HTML forms with no JavaScript and a dash of hackery.

  <form method="POST" enctype="text/plain" action="http://example.com">
  <input name='{"key1":"val1","params":{"input":"value","list":[],},"dummy":"' value='"}' hidden>
  <button>Submit</button>
  </form>

The important bit is to include a "dummy" key at the end of the JSON object, and an input value that closes the quotes and any open curl braces. That way the "=" character sent in the encoding of the form elements doesn't interfere with the meaningful JSON content.

There might be a clever way to get it to submit dynamic JSON that changes based on user input without JavaScript, but I haven't thought enough about it.

This technique is sometimes useful for CSRF attacks.