Frédéric recommends that mfenced be deprecated for the web. I've found that mfenced is a useful construct for two typographical reasons: 1) nested fences can be sized automatically to desirable heights, and 2) when equations are broken across lines, one fence knows the size of the other. OfficeMath<https://blogs.msdn.microsoft.com/murrays/2018/09/30/officemath/> makes use of these characteristics. It's true that, as with infix parsing in general, one can recover the mfenced nature of nested fences expressed as nested <mrow>'s and <mo>'s, but it's extra parsing. In fact, the OfficeMath MathML readers do just that in converting MathML to OfficeMath's internal format. An admittedly artificial example of nested fences that illustrates the auto sizing is
[cid:image002.png@01D4C2A4.E67920B0]
I entered this expression in Outlook, which supports OfficeMath. It may not display correctly on all email clients. It consists of 8 nested parentheses that increase in size as the nesting level decreases. The font is Cambria Math, which has parentheses with 8 size glyph variants. Still larger parentheses consist of 3-glyph glyph assemblies.
Thanks,
Murray
