This topic was already addressed with the link Russ provided.
XF follows the standard for the most part. Adjustments have to be made for code that is outside of XF's control. Social buttons, YouTube, and other products produce professional APIs which don't comply with standards of apps like W3's checker or PageSpeed. It doesn't make their work any less functional or professional.
Moreover, XF is HTML 5 which W3 clearly states their tool is only experimental in that area. phpBB is XHTML 1 so you are comparing apples and oranges. Perhaps come back when you find forum software that complies with the HTML5 standard.
When you have a development decision what do you want? A product that offers more functionality and is bug-free? Or one that complies with W3's tool?
There is zero benefit to being compliant with W3's tool which they clearly state is not an indicator of whether a page is good quality or not. The tool is great for you and 90% of the development community. It is not designed for the top professionals who fully know and understand the code at the highest levels, and make decisions to go in another direction in the interest of best serving their users.
Added Note: W3's HTML5 check currently doesn't support the canonical tag. If you have this tag on your page, which you should, then their app will say your HTML code is invalid. They approved the tag last week and plan to add it, but as of now it is considered non-compliant code according to their app. Should XF slow down and wait for W3C to catch up?
