Request for explanation: "the main element must not appear as a descendant of the td element" error message.

[ mailto:www-validator@w3.org?Subject=%20Add%20Subject%20Here&body=%0D%0A%0D%0ANOTE%3A%20Whenever%20possible%2C%20give%20the%20address%20of%20the%20document%20you%20were%20checking ] Hello, 

I've got this error message: "the main element must not appear as a descendant of the td element". 
The code was provided via Direct Input, and this code snippet allow to reproduce the error: 
<! DOCTYPE html > 
< html lang = "fr" > 

< head > 
< meta charset = "UTF-8" > 
< title > Table and zoning </ title > 
</ head > 

< body > 
< table role = "presentation" > 
< tr > 
< td > 
< header > Header content </ header > 
</ td > 
</ tr > 
< tr > 
< td > 
< main > Main content </ main > 
</ td > 
</ tr > 
< tr > 
< td > 
< footer > Footer content </ footer > 
</ td > 
</ tr > 
</ table > 
</ body > 

</ html > 

However, nothing in the HTML specification indicates that " the main element must not appear as a descendant of the td element": Content model for td is flow content and parent content model expected for main is also flow content (except for article, aside, footer, header and nav). So, what is the meaning of this error message? 

Thanks and regards 
Jean-Pierre