Another thing is that this standard does not allow setting an absolute size.. you can only use fractions, for example, 2/3 of normal size, which can still be relatively big if your normal size is big.
I also expect making things bigger for emphasis (or adding headings) would likely be more common than making them smaller (that's what I hope at least). Outside of things like mathematical notation where superscript/subscript might be useful (see this comment for some examples).
The problem with the web is that some websites use absolute units that might not scale well (like px) to define the sizes.
MAC addresses are easy to spoof in a Linux PC though. So you should be able to workaround that limitation if it's ever a problem.
In fact, in theory you can also spoof the IMEI in a phone via software, it's just that the software in phones is generally not as open and it would require rooting and some tweaking that not everyone is comfortable doing.