The documentation is unclear to me.
WordPress Core must validate
"All HTML pages should be verified against the W3C validator..."
Theme Development Standards say themes must validate
"Use well-structured, error-free PHP and valid HTML. See WordPress Coding Standards."
Links to http://make.wordpress.org/core/handbook/coding-standards/ which links to link above.
But, I can't find a straight answer to the issue of pluging standards. The following is the best I can find.
- "WordPress Coding Standards - General information about coding standards for WordPress development" links to the defunct WordPress Coding Standards
- WordPress Coding Standards says "The WordPress HTML Coding Standards are now located in the Core Contributor Handbook." (emphasis added)
- Core Contributor Handbook links to the quote at the top of this post.
That seems to suggest that plugins, like themes and core, must W3C validate, but it does not use strong, clear language the way that the theme development page unambiguously adopts the core coding standards.
Why do I care? Because I currently build my website by hand, but I plan to expand it from approximately 800 pages to tens of thousands of pages. I need a CMS to do that. I need my CMS to be predictable. While I prefer that plugins adhere to a standard, what I care about most is knowing whether or not I can expect a plugin on the WordPress repository to adhere to a standard. If I can't expect it, then that is ok: I will just need to test plugins more thoroughly.