W3C validation errors with WP plugins (many)

I am trying to improve the SEO performance of my WordPress site, but I’ve hit a wall with almost 40 W3C validation errors, most of which are beyond my control (short of hacking the code of WP plugins).

The issue is that plugin authors don’t seem to like bothering to properly close their tags in the HTML code. Or is there some specific reason why they would not want to do this?

For example:

…ontent/plugins/mappress-google-maps-for-wordpress/css/mappress.css?ver=2.40.7″> (Mappress Pro)
…t” href=”/wp-content/plugins/pinterest-pin-it-button/css/public.css?ver=2.0.3″> (Pinterest Pin It Button)
…f=”/wp-content/plugins/really-simple-popup/css/hs_rsp_popup.min.css?ver=3.8.1″> (Really Simple Popup)
…” href=”/wp-content/plugins/wp-ultimate-search/css/visualsearch.css?ver=3.8.1″> (WP Ultimate Search)
…ink rel=”stylesheet” href=”/wp-includes/js/thickbox/thickbox.css?ver=20131201″> (Thickbox)

These are just a few examples. Most plugin code seems to cause similar validation errors. Has this been discussed in the past? Is there some reason it’s still happening? Does no-one else validate their HTML?

Sorry, I know I’m being a bit sharp here, but it’s frustrating.