Simple, accessible, unstyled tabs built entirely in vanilla JavaScript. Based on the APG Tabs pattern.

npm add @acab/tabs

Import the tabs and register it.

import TheTabs from '@acab/tabs';

window.customElements.define('the-tabs', TheTabs);

Inside the custom element, define the initial markup consisting of role=tablist, role=tab and role=tabpanel. Add aria-selected='true' to the tab you initially want to be active, and add hidden to the panels of the other tabs.

<the-tabs>
	<div role='tablist' aria-label='…'>
		<button role='tab' aria-selected='true'>…</button>
		<button role='tab'>…</button>
		<button role='tab'>…</button>
	</div>

	<div role='tabpanel'>…</div>
	<div role='tabpanel' hidden>…</div>
	<div role='tabpanel' hidden>…</div>
</the-tabs>

Now style it however you want! The [role=tablist]/[role=tab]/[role=tabpanel] and [aria-selected=true] selectors can be referenced in CSS.

[role='tablist'] {
	/* … */
}

[role='tab'] {
	/* … */
	
	&[aria-selected='true'] {
		border-bottom: 2px solid;
		/* … */
	}
}

[role='tabpanel'] {
	/* … */
}