I really like ThymeLeaf (TL), and while playing with PetClinic (Spring+TL) just added a <hr> close to the end of the welcome.html file as follows, and an Exception has been thrown:
</div> <hr> <----------------------Added this here
SEVERE: El Servlet.service() para el servlet [petclinic] en el contexto con ruta [/petclinic] lanzó la excepción [Request processing failed; nested exception is org.thymeleaf.exceptions.TemplateInputException: Exception parsing document: template="welcome", line 31 - column 7] con causa raíz
Check your template mode. If you are using XHTML, that syntax is invalid (should be "<hr />"). In Thymeleaf's "HTML5" template mode, only xml-valid syntax is allowed, so you would need "<hr />" too. If you want to be able to use non-XML-valid syntax, you should be using the LEGACYHTML5 template mode.