Ok so after checking carefully, it is obvious that YaCy web interface pages are declared as Strict XHTML 1.0 according to their DTD, but indeed have been long time using many HTML 5 features and syntax.
You can easily check this with the W3C Markup Validation Service
on your YaCy peer main search page, and compare how many validation errors are reported in XHTML 1.0 Strict validation mode (automatically detected) and forced HTML5 :
- XHTML 1.0 : 56 Errors, 56 warning(s)
- HTML 5 : 1 Warning and 1 Error
So let's fix this!