Any child html (that is not assigned a slot) will be rendered inside the mockup document.
Assign an element's slot attribute to "tab" to render that child as a tab element.
If a child element is assigned to the tab slot, and has a class of "selected" that tab will appear selected. If no tab has a "selected" class, the first tab will be assigned that class.
Use the url attribute to set the path in the mockup.
`<browser-window>` elements can be nested.
Add a class of 'windows' to add mockup form buttons from that operating system.
Add a class of 'macos' to add mockup form buttons from that operating system.
Using the tabs and collapsed-tabs attributes injects randomly generated tabs into the mockup.
Set the slot attribute on child elements to "tab".