Most of that validation info is not even referencing the plugin but the wordpress theme and images on site. A completely different thing ( your references arenot related to plugin - and the plugin working or not )
99% of WordPress themes and plugins are not created for HTML Validation.
Let's NOT confuse other people here...
1. The plugin works ( With or without html validation )
2. Your site will work
3. This is a free plugin
4. Html validation while nice isn't an issue really
( heck 99% of people don't even know what html validation is )
Having a plugin on your site or a theme that doesn't pass html validation isn't going to kill your site and it isn't going to slow your site down or cause issues with ranking in google. As billions of sites are not html validated.
Heck let me give you an example - go to google.ca and type the word
Into the search field, the site that is near the very top of the page ranking for the term cars
Has over 307 errors and 40 warnings
Do you see it effecting how fast it loads?
Do you see it effecting how google ranks it?
As I've said before, html validation is NOT important, What is important is making sure a theme, site, or plugin works ( and that I can say with confidence my free coupon plugin does )
To uninstall it because it didn't validate is a bit extreme in my opinion.
At the end of the day - the plugin works and it's free