With docutils v0.17 a lot of html markup has been changed (see below) what cause a lot of problems in CSS from Sphinx and other Sphinx extensions & customizing. For the first this fix pins to previous v0.16. In sphinx 4.0 these problems will be addressed [2] and we can relax (drop) in the requirements-dev. HTML5 writer [1]: Use the new semantic tags <main>, <section>, <header>, <footer>, <aside>, <figure>, and <figcaption>. See minimal.css and plain.css for styling rule examples. Change the initial_header_level setting's default to "2", as browsers use the same style for <h1> and <h2> when nested in a section. Use HTML text-level tags <small>, <s>, <q>, <dfn>, <var>, <samp>, <kbd>, <i>, <b>, <u>, <mark>, and <bdi> if a matching class value is found in inline and literal elements. Use <ins> and <del> if a matching class value is found in inline, literal, or container elements. New optional style responsive.css, adapts to different screen sizes. New option embed_images. [1] https://docutils.sourceforge.io/RELEASE-NOTES.html [2] https://github.com/sphinx-doc/sphinx/issues/9056 Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>