[smnm]

In 'Live preview' I casually inspected the the first button, and it has 63 classes, wow

  <button class="relative isolate inline-flex items-center justify-center gap-x-2 rounded-lg border text-base/6 font-semibold px-[calc(theme(spacing[3.5])-1px)] py-[calc(theme(spacing[2.5])-1px)] sm:px-[calc(theme(spacing.3)-1px)] sm:py-[calc(theme(spacing[1.5])-1px)] sm:text-sm/6 focus:outline-none data-[focus]:outline data-[focus]:outline-2 data-[focus]:outline-offset-2 data-[focus]:outline-blue-500 data-[disabled]:opacity-50 [&amp;>[data-slot=icon]]:-mx-0.5 [&amp;>[data-slot=icon]]:my-0.5 [&amp;>[data-slot=icon]]:size-5 [&amp;>[data-slot=icon]]:shrink-0 [&amp;>[data-slot=icon]]:text-[--btn-icon] [&amp;>[data-slot=icon]]:sm:my-1 [&amp;>[data-slot=icon]]:sm:size-4 forced-colors:[--btn-icon:ButtonText] forced-colors:data-[hover]:[--btn-icon:ButtonText] border-transparent bg-[--btn-border] dark:bg-[--btn-bg] before:absolute before:inset-0 before:-z-10 before:rounded-[calc(theme(borderRadius.lg)-1px)] before:bg-[--btn-bg] before:shadow dark:before:hidden dark:border-white/5 after:absolute after:inset-0 after:-z-10 after:rounded-[calc(theme(borderRadius.lg)-1px)] after:shadow-[shadow:inset_0_1px_theme(colors.white/15%)] after:data-[active]:bg-[--btn-hover-overlay] after:data-[hover]:bg-[--btn-hover-overlay] dark:after:-inset-px dark:after:rounded-lg before:data-[disabled]:shadow-none after:data-[disabled]:shadow-none text-white [--btn-bg:theme(colors.zinc.900)] [--btn-border:theme(colors.zinc.950/90%)] [--btn-hover-overlay:theme(colors.white/10%)] dark:text-white dark:[--btn-bg:theme(colors.zinc.600)] dark:[--btn-hover-overlay:theme(colors.white/5%)] [--btn-icon:theme(colors.zinc.400)] data-[active]:[--btn-icon:theme(colors.zinc.300)] data-[hover]:[--btn-icon:theme(colors.zinc.300)] cursor-default" type="button" data-headlessui-state="">Button<span class="absolute left-1/2 top-1/2 size-[max(100%,2.75rem)] -translate-x-1/2 -translate-y-1/2 [@media(pointer:fine)]:hidden" aria-hidden="true"></span></button>

[tipiirai]

Is this called a single class in Tailwind: ”px-[calc(theme(spacing[3.5])-1px)]”?

[smnm]

Don't know about Tailwind, but in DOM API it is definitely a single class.