I aim towards making everything I build for the internet HTML valid. Web standards exist to promote improved accessibility, performance and integrity of completed websites. I beleive HTML validation is a good benchmark of website quality. It’s sensible not to ignore the errors and to take a more proactive approach towards fixing things.
Making a website pass validation tests isn’t extremely difficult (if you’re coding a site by hand or using a publishing platform that advertises itself as W3C standards compliant). Unfortunately there are some RapidWeaver addons that use invalid HTML code, and these can subsequently “drag a good website under”. Hopefully it is something more RapidWeaver developers will learn about and work harder to resolve in future updates.
From a user standpoint, there is not much you can do. I certainly would not recommend that you start editing any code. It would be better to contact addon developers and request updates to addons that might be throwing validation errors. If it’s filed as a bug, it is more likely to get fixed. Or if you get a rather muted response (e.g. "validation doesn’t matter"), then it might be wise to look at switching towards using addons from other companies.
In all fairness to developers, the HTML5 specification has been painfully slow to get implemented. It has taken about a decade to get HTML5 to where it is now! A number of changes and u-turns have happened along the way. It has been difficult at times for developers to comply with everything; especially if other frameworks, APIs or web browser compatibility are pulling them in a different direction.
For example, for a long time RapidWeaver inserted rel=“self” into links (invalid code) and although that bug has since been fixed, the error still remains in any content migrated into RapidWeaver 6. It’s not something addon developers can fix, even though it looks like a theme or stack issue. Deleting the link and recreating it again is the only way to fix it. But that’s a lot of hard work and rel=“self” doesn’t cause any serious issues.
The automated W3C validator is very fussy and will flag even the most insignificant issues (like social media meta tags it does not recognise). But ultimately lines have to be drawn somewhere, and all designers / developers are subjected to the same specifications and rulesets. I do strongly believe we need specifications to work towards and the W3C validator is a very good tool for testing with.