Mark the end of a definition with \qed

This is a very simple solution to start with.  For PDF output, there are definitely more advanced variants to consider, but the question is what would also work well (look good) with LaTeXML.

preamble.tex

@@ -137,20 +137,7 @@
 
 \newcommand{\bibitemtitle}[1]{\emph{#1}}
 
-% henrikt-ma: Introducing semantic macro for 7 definitions spread out over two chapters
-% that for unclear reasons use their own style, including numbering.  Sorting out if numbered
-% definitions should be used more or not at all is part of issue #2570.
-%
-% Definitions are numbered, but are not suitable for use with \autoref, as \autoref is not aware
-% of our custom definitions.  (They say there are better alternatives to \autoref, more suited
-% for this kind of customization...)  To avoid accidental use of \autoref, a definition should
-% also avoid use of \doublelabel, and instead use \definitionlabel.  Instead of \autoref, the definition
-% should be referenced using \definitionref
-%
-% If we'd like to have definitions spanning over several paragraphs, we should use an environment
-% instead of a command, but then we also need to find a way to visualize where the definition ends;
-% alternatives that come to mind include boxes, indentation, a right-flushed black square, but the
-% big challenge would be to find something that also works well with LaTeXML.
+% Environment for definitions.
 \usepackage{amsthm}
 \newtheoremstyle{mlsdefinition}
   {\topsep}   % ABOVESPACE
@@ -162,8 +149,13 @@
   {.75em}     % HEADSPACE
   {Definition~#2. \firstuse{#3}}        % CUSTOM-HEAD-SPEC
 \theoremstyle{mlsdefinition}
-\newtheorem{definition}{Definition}[chapter]
-\newcommand{\definitionautorefname}{definition}
+% In order to show where the definition ends, we put a \qed at the end.  This can't be done using \newtheoremstyle,
+% so instead we make the 'definition' environment a wrapper around the amsthm-base environment.
+\makeatletter
+\newtheorem{definition@inner}{Definition}[chapter]
+\newcommand{\definition@innerautorefname}{definition}
+\newenvironment{definition}{\begin{definition@inner}}{\qed\end{definition@inner}}
+\makeatother
 
 \setcounter{secnumdepth}{5}
 % Note: Toc changed for appendex