Hi @masgo,
maybe there is something in the making for you. I’ve dealt with this issue for quite some time and have also talked to people who construct full HTML-Reports “manually” in KNIME.
To overcome this shortcoming I developed a set of nodes which leverage Asciidoctor for the output. Asciidoc is a human-readable markup language comparable to Markdown with a more strict definition and is supported by Asciidoctor, GitHub, Gitlab and a few others. This language allows to gradually add simple text sections of the final document in Asciidoc and finally render it to .html or .pdf (Asciidoctor also supports other formats like epub or Docbook but thats untested by me so far).
Let me give you a sneak preview from my development workflows:
The three Java snippets add building blocks of the document and the GroupBy Node concatenates everything to Asciidoc-Text in the bottom of this post. The two AsciiDocWriter Nodes export the raw text to either an .html-Document or a .pdf (Asciidoctor also supports epub and Docbook but I haven’t tested that so far).
The results are as follows:
Rendered HTML-File
Rendered PDF-File
For a more complex example of whats possible with Asciidoc see this file
That’s a rather short example but it definitively also works for longer documents. I’ve used it to construct a report about published preprints where the data fore each paper has been added in a loop and then a final document has been rendered. We used to run this workflow in a dockerized KNIME triggered by Jenkins and send out the report by email but it should be somewhat straightforward to also publish the generated files to a webserver.
So far the Nodes have only been used internally but I am currently working with Daniel and Philipp from NodePit to release these nodes for the general public. I hope to get out a beta version by the end of the year at the latest. So if that sounds interesting be sure to follow my posts and the NodePit website for further details… 
Cheers,
Lars
And last but not least for reference the raw source text for both outputs:
= Asciidoc Nodes Example
:description: This document is a quick example about what's possible with the AsciiDoctor Nodes for KNIME
:keywords: AsciiDoc, Asciidoctor, pdf, theme, themeing
:page-description: {description}
:page-keywords: {keywords}
:page-layout: docs
:page-javascripts: [view-result]
:experimental:
:table-caption!:
:example-caption!:
:figure-caption!:
:toc: macro
:icon: font
:icon-set: fab
:imagesdir: D:/Temp
The AsciiDoc Nodes are a new way of reporting in KNIME. It is easy to construct reports in multiple blocks and then convert them to an .html or .pdf-Document
It's also easy to integrate images that have been previously rendered to .jpg, .png or .svg!
.Asciidoctor image
[#AsciidoctorLogo,link=https://asciidoctor.org/]
image::Asciidoctor.png[Asciidoctor,318,159]