I see that 0.27 version has started to report the following warnings:

WARNING:html5validator.validator:"file:/home/travis/build/php-coder/mystamps/src/main/webapp/WEB-INF/views/error/403.html":1.16-5.73: info warning: This document appears to be written in Danish but the "html" start tag has "lang="en"". Consider using "lang="da"" (or variant) instead.
"file:/home/travis/build/php-coder/mystamps/src/main/webapp/WEB-INF/views/error/500.html":1.16-5.73: info warning: This document appears to be written in Danish but the "html" start tag has "lang="en"". Consider using "lang="da"" (or variant) instead.

Here are these files:

• https://github.com/php-coder/mystamps/blob/be8d102327bcc68ca256bde8aed489886bd86835/src/main/webapp/WEB-INF/views/error/403.html
• https://github.com/php-coder/mystamps/blob/be8d102327bcc68ca256bde8aed489886bd86835/src/main/webapp/WEB-INF/views/error/500.html
  They're in English and not in Danish so these warnings are false positives.