Restrict to designing purely with CSS styles/properties, match browser's rendering appearance
As has been noted for this to be a good web app it will need to fully embrace css and render it accurately.
Therefore it would be useful to tag objects or artboards to display in strict mode - exactly according to css properties - or to display in lets say design mode to allow freeform placement of objects which would require additional markup in development to work properly.
Per Mattsson commented
I know XD is not only for web but my guessing is that a lot of us are using it for that.
That's why I'm surprised that XD does not cover even 1996 basic css-styling.
Today, I don't think it's exaggerated to wish for
1) support of whatever we can do in CSS3
2) the ability to style objects not only thru GUI but also using CSS styles with css-selectors on objects. Think of it as a document overall CSS-file, not only for a single artboard. Generally a better integration between css and XD would be very useful.
3) the ability to export an artboard as css + svg + html with the selectors I've set. We can already copy css but that is a bit tedious with an object heavy design. It's a novelty, not for production.
Maybe a better integration with that old Dreamweaver would be a possible way? DW already has the mechanics for most of the css+html workings. That would open up use of XD/DW closer to final production, not just as a tool for ideas.
We can do scss, @font-face linking encodings , etc later in whatever we use for publishing
To be a killer web design tool this app needs to have a full css toolset at least as far attributes that are key to design and presentation and artboards that render accurately according to css properties.
This should include definable breakpoints that are detected by artboards and the ability link objects across artboards under global styles as has been noted elsewhere to show responsive variation.
The ability to export css, not necessarily as a complete production ready stylesheet but in a form that i can extract assets from for production instead of recreating everything.
The functionality of this program should match what's possible in HTML 5 EXACTLY.