I was looking into this too about 8 months. I stumbled across a blog post written by Martin Hawskey. I followed the guide here and I was able to set up a HTML form posting to a spreadsheet.
Effectively you set up a published web app inside the spreadsheet that can receive the data. To get around the CORS issues you target a hidden iframe on the page. I would replicate the code in this post but there is a fair bit of it.
DEMO
I'll provide some advice that I wish I was given when I started looking at this. If you can... try and set up a PHP server you can use. Posting the data is a lot easier and flexible. I now use Zend GData religiously at work and wish I had found it sooner :)
EDIT
Marting Hawskey has updated this to support a AJAX submission without the use of a hidden iframe. See here.