w3m does rrecognize <input type="button">, but does not recognize <button>. Both are valid.
Test using this html:
<html>
<head></head>
<body>
<form>
<input type="button" value='input' />
<button type="button">foo</button>
</form>
</body>
</html>
Fixed in Debian w3m 0.5.2-10.
The patched snapshot can be downloaded from:
http://anonscm.debian.org/gitweb/?p=collab-maint/w3m.git