[geezerjay]

> You accidentally apply a global style, and your entire UI is broken.

That's only a concern if your UI elements depend on global styles, right?

Meanwhile, UI toolkits such as Qt use CSS (actually, Qt Style Sheets) to style UIs

[dmitriid]

> That's only a concern if your UI elements depend on global styles, right?

No.

   div {
      padding: 15em;
   }

This will affect all of your styles. CSS is a flat global namespace where you fight to style your local components by overriding rules with increasing specificity.

[HelloNurse]

Given the abundance of div elements in modern HTML, a CSS rule to set padding in all of them can only make sense in an unlikely minority of abnormally simple and constrained pages. Setting a huge, fixed 15em padding is also highly suspect.

You need to "fight" only in poorly designed systems; UI elements can pretend to have a hierarchical namespace and avoid all interference with other components, and pages can be designed systematically (e.g. box-sizing border-box vs. content-box, padding vs. margin, flex...) to simplify CSS rules.

div.arbitrary-ui-library-prefix-radio-button-group { padding:0.5em; border: 0.1em dotted rgba(0,0,0,0.3); }

div.arbitrary-ui-library-prefix-huge-padded-box-container { padding:15em; }

[d0100]

> You need to "fight" only in poorly designed systems;

So pretty much most systems, especially general web Dev and frameworks

[geezerjay]

> This will affect all of your styles.

Your example affects all div elements which are not styled. If you also include class and/or ID specific definitions then your padding setting doesn't cascade and those are not affected. That's the point of CSS.

> where you fight to style your local components

That's not true. Style changes do cascade but they only cascade where the designer intends them to cascade. If a designer specifies that all div components shall use the same padding then you can't complain that all div components are using the same padding.

[majewsky]

> UI toolkits such as Qt use CSS (actually, Qt Style Sheets) to style UIs

This statement makes it sound like every Qt application uses CSS. That's not true. CSS is offered as a way to apply corporate branding to an application. I'm not aware of any FOSS Qt applications using CSS.

[geezerjay]

> This statement makes it sound like every Qt application uses CSS. That's not true.

The applications that are ok with the default style don't need to specify style. Nevertheless Qt is an UI toolkit that supports CSS styling.

> I'm not aware of any FOSS Qt applications using CSS.

And that's ok. Nevertheless I've already worked on a couple of Qt projects that used Qt Style Sheets extensively. In fact, that's pretty much the norm in any non-desktop project.