HTML: Experimentally switch to HTML5 output

src/main/webapp/WEB-INF/stylesheets/db2xhtml.xsl

src/test/resources/11sqw.0.html

src/test/resources/mjr3.0.html

src/test/resources/mr23.0.html

src/test/resources/mr47.0.html

src/test/resources/msdd.0.html

src/test/resources/n2kf.0.html

src/test/resources/nbvb.0.html