Here's a simple issue that I've never figured out:

1. Create an svg

2. Draw an svg rectangle in there

3. Draw an svg text "Hello World" so that it fits inside the rectangle.

4. Make sure #3 above works regardless of which platform/browser you are using.

Now, I don't see it as a catastrophic problem that I can't (easily) automate everything up to step #3. I mean, maybe I could by iterating over various sizes inside an offscreen HTML5 canvas and measuring the results or something. But it isn't the end of the world to, say, just guess and check in a browser until I hit the right pixel size.

Problem is, that rendered result tells me absolutely nothing about what will render on other platforms. Maybe OSX will eat less horizontal width. Hell, maybe some troglodyte who lives deep in the guts of Gnu font rendering stack makes a change that accordions out the horizontal width so it's way out of whack with Windows and OSX. There's no spec that says a troglodyte can't do that. (This all holds for fixed-width fonts the same as variable width, btw.)

It's weird because I see all these SVG frameworks that have demos with, say, a graph where text looks to be aligned so that the end of a label doesn't overlap with a vertical line. But there's never a caveat given-- if there were, it should be that SVG cannot guarantee there won't be an overlap there, because it cannot control the underlying font engine that way. You literally have to check with your eye on every platform that you want to support[1].

Edit: clarification

Edit:

[1] You can't even simply check for each different font stack-- you have to check every extant version of that stack. Because again, a font stack can make a lot of arbitrary changes from version to version.