Vertically center an image for pdf export

[Josh Dawson]

I'm using renderModes to create X(HT)ML content to be exported to a pdf. Knowing which css features are supported by Confluence Storage Format is a struggle (especially with the web's infamous vertical centering problem).

It seems `flex` and `inline-block` display formats are unavailable. As is `absolute` and `relative` positioning. I've also tried `translate` and even `ac:vspace`.

Perhaps I'm using `ac:vspace` wrong - `<ac:image ac:vspace="100px"><ri:url ri:value="url" /></ac:image>`.

If anyone has other ideas or has successfully conquered this particular dilemma, please divulge.