I render the "width" attribute of an image in PHP like this:
width=\"".$width_out."\"
The variable $width_out is calculated and passed the PHP function round() to get rid of the decimals.
When I validate the page with both validators (also validator.nu) the follwoing error message is generated:
<<<<
 Line 462, Column 156: Bad value -2 for attribute width on element img: Expected a digit but saw - instead.

…/index-images/money.jpg" width="-2" height="1"  alt="m" title="Index: m" /></a>

Syntax of non-negative integer:
    One or more digits (0–9). For example: 42 and 0 are valid, but -273 is not. 
<<<
But rightclick mouse source code gives :
<<<<<<<<<<<<
index-images/money.jpg" width="257"
<<<<<<<<<<<
With other words the correct value for $width_out,
Your validators source code is different to firefox's source code. 
I tried the function intval() and number-format(), no results.
The only way to trick your validator to validate my page is using:
str_replace ('-','',$width_out)
But your validators source code keeps on rendering the number 2 instead of 257!
The renders well in both caes. I am just curious what this is.
Alexander