XAML is far easier for app layout than HTML/CSS. It's very easy to do responsive design. We do have a full screen layout for those that want it, and it's what you get in Tablet Mode.

    <Grid>
        <TextBlock VerticalAlignment="Center"/>
    <\Grid>

I meant resizing the startmenu with the mouse by grabbing the border and moving the mouse.

It's not that kind of "responsive design" we know from HTML. The startmenu only snaps to at least three sizes (vertical) and doing so with little UX feedback animation. It could be done better even with XAML (and it would be trivial with CSS3), but it was probably a design choice - it's something for Raymond Chen to write about in ten years.

(And changing back from tablet-mode startmenu to desktop-mode startmenu is a bit hidden in the icon next to the clock and takes some time to locate the setting.)

Yeah XAML layouts have always seemed so much cleaner than any CSS layout system. Was hoping that css-grid would have gained more traction over flex.

Given the choice, I'd choose XAML over HTML/CSS for app layout/design in a heartbeat.

I think it all goes back to what each language was designed for and people hacking things into HTML/CSS to get it do things it wasn't designed for.

XAML is much better at solving the UI issue on the desktop. Notably, it allows you to use binding logic to ensure your data model for your application doesn't carry all the weight of the UI logic (such as coloring, resizing, en/dis-abling controls, and etc). So, your XAML contains all the logic that deals with UI and your data model (view-model) contains all the values to be displayed (and possibly some of the business logic tied to those values).