Thank you for the suggestion – you are correct, HTML code is far too limited, and as much as I like the Admonition plugin I’m not sure how I could change it’s appearance to wrap a box around my text.
The closest possible solution I came across can be found here: Blockquotes with different border/background colors - Help - Obsidian Forum
blockquote.red { border-color:#f00; background-color:#f001; }
blockquote.green { border-color:#0f0; background-color:#0f01; }
blockquote.blue { border-color:#00f; background-color:#00f1; }
blockquote.magenta { border-color:#f0f; background-color:#f0f1; }
blockquote.cyan { border-color:#0ff; background-color:#0ff1; }
blockquote.yellow { border-color:#ff0; background-color:#ff01; }
blockquote.pink { border-color:#f9f; background-color:#f9f1; }
Where there are different styles applied to the blockquote – this leads me to suspect something similar could be achieved with the right technical knowledge… but I don’t know enough about CSS to know for sure.