Classes list

Every class Button recognizes, straight from the @flickui/css source.

ClassPurpose
.btnBase class — required on every button
.btn-labelWraps the visible text (hidden automatically during .is-loading)
.btn-leading / .btn-trailingIcon slots before/after the label
.btn-spinnerLoading-state spinner overlay (shown only while .is-loading is present)
.btn-fullStretches the button to 100% width of its container
.btn-icon-onlySquare, padding-free sizing for an icon-only button
.btn-actionMarks an <a class="btn"> so the UA's visited-link color is suppressed
.toggle-targetBehavior hook — opens/toggles the component referenced by aria-controls/href
.action-close / .action-cancel / .action-confirmBehavior hooks for buttons inside a dialog/overlay
.is-loadingRuntime state — hides the label/icons and reveals .btn-spinner