Signs of ugly HTML

Severe

Use of FONT tag
Automatically-generated IDs, eg. Table_01, style3, style4
Multiple HTML, TITLE tags

Pretty Bad

A which has no content inside its link (displays as no link) unless its a A NAME
BR or P followed by DIV or /DIV
BR followed by BR followed by BR
Use of cellpadding, cellspacing, bgcolor, bordercolor or background
Two IDs repeated on the same page
TDs which only contain IMGs (indicate likelihood of table-based layout)
STYLE outside of HEAD

Moderate

IMG with a width or height of 1
Use of NBSP
Use of align, valign

Trivial

Use of B instead of STRONG
Use of I instead of EM

Warnings

INPUT elements outside of FORMs