Widoco output does not pass W3C HTML Validator

[lgleim]

Describe the bug The output of Widoco does not pass validation by the official W3C HTML Validator @ https://validator.w3.org/nu/

To Reproduce Steps to reproduce the behavior:

1. Run

mkdir -p widoco
wget -q -P widoco https://reshare-dtc.github.io/reshare-ontology/0.3/ontology.rdf
wget -q -O widoco/widoco.jar https://github.com/dgarijo/Widoco/releases/download/v1.4.17/java-11-widoco-1.4.17-jar-with-dependencies.jar
java -jar widoco/widoco.jar -ontFile widoco/ontology.rdf -outFolder widoco -webVowl -rewriteAll -excludeIntroduction

2. Upload the .html documents to https://validator.w3.org/nu/
3. Observe errors

Expected behavior Widoco should output Valid HTML documents

Screenshots Snapshot of some of the observed errors/warnings:

Additional context While non-compliance with the official W3C HTML Validator may arguably not be a big issue for many users of Widoco, it is an important one making it harder to use it for documents to be published through the W3C. Publishing Widoco-generated documentation there can have significant impact and improve the accessibility of ontologies, however, all documents must be valid HTML according to the https://validator.w3.org/nu/ validator.

[dgarijo]

Thanks! back in the day everything was validated, but new errors appear as time goes by, it's logical to have things change. Please feel free to fix some of this in a pull request :)