To be able to run the net core server for HTML 5, all transformations has to be converted from ‘XslTransform’ to ‘XslCompiledTransform’. You can do this using e.g. linux or, or ubuntu developer console at windows 10 or probably even git bash console at windows 10 using the following command:
find -path "*/Transformation/*" -type f -exec sed -i 's/engineType="XslTransform"/engineType="XslCompiledTransform"/g' {} \;