Okay, way back in Sigil 0.7.1 that feature for using a ';' ended up causing an empty html bug and so was removed by this commit from over 13 years ago.
https://github.com/Sigil-Ebook/Sigil...bc8313df0ba6e2
That was before we took over Sigil. If you found that ';' feature documented in a current Sigil User guide, we will have to fix the user guide to remove that feature. The tooltips in Sigil no longer mention that use of ';'.