tex4ebook: invalid attribute value error

Question

The following MWE shows the error with tex4ebook named in the question, when running epubcheck. It gives (in the MWE) 3 identical error messages:

\documentclass[a4paper]{report}
\usepackage[cmintegrals,cmbraces]{newtxmath}
\usepackage{ebgaramond-maths}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\usepackage[rm,sc,small]{titlesec} 
\renewcommand{\thesection}{\arabic{section}}
\titleformat{\section}[display]% 
{}% 
{\Huge\centering\textbf{\thesection}}%
{0.2\textheight}{}%
[]

\begin{document}
    
\clearpage
\section{}
Some first little poem.
    
\clearpage
\section{}
Some second little poem.

\clearpage
\section{}
Some third little poem.
\end{document}

First I thought it has to do with the empty sectioning text, but that is not the problem. The error remains when giving a section-title.

Answer 1

This is caused by the sectionid filter from make4ht. It creates unique ID for each section based on it's name. When the section name is empty, it sets the empty ID for the first section, and then adds numerical suffixes, because it detects duplicate name (the empty name). epubcheck reports these epmty and numerical IDs as errors.

I've fixed this in the development version of make4ht, it now adds a _ prefix, so the ID should be valid. If you don't want to use the development version, you can disable make4ht filters using this command:

$ tex4ebook -f epub-common_domfilters  filename.tex

This should prevent creation of IDs for sections, but it can lead to other problems, depending on your document.