I noticed two differences between cmark-gfm and md2html table rendering.
I extracted all examples from test/spec-tables.txt and ran them through
cmark-gfm 0.29.0.gfm.13 and md2html 0.5.2. looking for differences.
The following two examples showed differences.

In this example cmark-gfm renders an HTML table
while md2html does not (consistently with its spec).

1. The table cannot interrupt a paragraph.

Lorem ipsum dolor sit amet.
| Column 1 | Column 2
| ---------|---------
| foo      | bar
| baz      | qux
| quux     | quuz

In this example cmark-gfm renders

<td>`foo</td>
<td>bar`</td>

while md2html renders (consistently with its spec)

<td><code>foo     | bar</code></td>
<td></td>

2. However pipes which are inside a code span are not recognized as cell
   boundaries.

Column 1 | Column 2
---------|---------
`foo     | bar`
baz      | qux
quux     | quuz

Since md2html is behaving per its specification I don't think the differences
can be considered defects, though, in practical terms, mark-gfm is doing a
better job with those two tables. Your thoughts?