The broader issue here, which I’ve been dancing around for years now, is that there’s no good compromise between (Z-machine) “here’s a tiny TTY device / fixed-width grid” and (Twine) “you got a damn web browser, everything is HTML and CSS, jQuery is installed, you’re on your own, pal.”
There may exist a sweet spot in between those extremes. I even have some notes on finding it. What’s certain is that the stylehint API that I came up with in 1997 (for Glk/Glulx) is not that sweet spot. It’s terrible and almost completely useless, for a variety of reasons. If there were a better option, it wouldn’t be compatible with stylehints as they exist today (to the extent that they exist today, which is not all that much). 
So, with apologies, no. Stylehints ain’t coming back. The answer to the question about Glulx Image Centering is that Glulx doesn’t support image centering.
Now, I realize this is a frustrating answer on at least two axes:
Legacy Z-machine support is a real use case and Parchment wants to do it. Extending Quixe/GlkOte for Z-machine features (primarily color) is a reasonable thing to do. Those changes aren’t going to be absorbed back into the spec, but they can exist in Parchment and Lectrote as a separates set of files.
Stylehints can’t do much but they can do centered text and inline images. As we see, people rely on this feature already, or want to. As a narrow use case, it’s pretty high return for a small amount of effort and complexity. So there’s an argument for keeping that much of the old API un-deprecated.
Sorry about the long and inconclusive rant.
Footnotes, because I’ve been turning this over in my head since last night…
(1: Although that notes file has a last-touched timestamp of February 2012.)
(2: At a quick glance: Stylehints can only deal with integers. So no font names, no units on dimensions, no fractional numbers, for a start. Also, stylehints don’t encompass the difference between block (paragraph) and span (letter) styles. So no way to represent italics in a centered paragraph. Still less any fancier layout such as toolbars, menus, full-layout status windows, etc. More abstractly, stylehints are way too crude to balance the requirements of interpreter style control (app-wide font preferences, window size, etc) with the requirements of the author/designer. When I wrote down the initial ideas, I handwaved a lot and said “compromises are possible”, but there was never any plan for actually doing it.)
(3: The better way which exists today is Vorple, which I think demonstrates my point – it works by letting you get at the HTML/CSS directly.)
(4: To dig into this a bit: text centering does not clash with interpreter preferences/typography the way per-span color text does.)