@Mike is on to something that I discovered after hunting around a bit. It may be that had a defective Helvetica font - but having now removed all helvetica fonts (+rebooted) I still get the same problem.
BUT - it is perhaps relevant for TW after all because others, out of TW context, have had the same problem occurring in Chrome and IE for various sites- regarding Helvetica (and variants) specifically. For instance, the page
ted.com (TED talks) also displays badly. Here is an attempted sum-up explanation from the comments section in
this page;
The thing with Helvetica is that it looks nice on Mac (and it’s a default Mac font that is pre-installed), and it’s similar to Arial, which looks nice on Windows (and is pre-installed on Windows).
So, very often, font declarations are set to “Helvetica, Arial” to ensure that it looks good (or at least similar) on both Mac and Apple: Mac will show it as Helvetica, and Windows will show it as Arial.
In most cases.
When a Windows user has Helvetica installed, then it will also show Helvetica on the site. And here’s the problem: it’s not always the right Helvetica. I’ve seen a lot of cases where a Windows users think they have the standard Helvetica installed (the one that’s similar to Arial), but in fact it’s some form of Helvetica that’s way wider.
As stated, I've removed all helvetica fonts from Windows now so I'm not quite sure what font is actually showing now (or maybe browsers have locally installed fonts, including helvetica variants?) But, in TW when I change font from default it then displays properly both in Chrome and IE; that is, I change font via Ctrlpnel > Appearance > ThemeTweaks. Changing the first default font-family, "Helvetica Neue", into anything else, makes the TW display the text properly including when using plain Helvetica. Thus I conclude it is the specification of "Helvetica Neue" that is causing the problem. And this goes for both Chrome and IE. I think it may be a good idea to put this on record / remember it.
I'm not sure why it was triggered in Chrome only now. As mentioned, I've had it in IE for a long time (years?) but I don't just IE so I ignored it. Maybe the Win8.1 update or perhaps some update in Chrome.
I am currently looking for some alternative way to deal with this (extentsions, settings etc). Just maybe it's worth considering not using "helvetica neue" as primary default for TW? At least if anyone else has this problem too. BTW, stumbled over this:
Arial vs Helvetica
<:-)