The {tab} tags are not being converted

The Tabs plugin is not published

Check if the Tabs plugin is published. Go to Extensions >> Plugin manager and search for the system plugin System - Regular Labs - Tabs. Then publish it.

There is markup on the {tab} tags

It is fine to give styles to the titles within the {tab} tags, like: {tab title="My Title"}
But giving styles to other parts of the tag, or the entire tag, can cause problems, like: {tab title="My Title"}

If you have copy/pasted code directly into your editor, you might also have copied along hidden HTML tags. So this will also cause the {tab} tags to break:

{<span>tab</span> ...}

Remove the extra styling or other html tags around and in the {tab} tags that might cause problems.

Tabs are not working on all browsers

There are javascript errors

Javascript errors might and usually will cause the tabs to disfunction. You probably have a Mootools or jQuery conflict on your site. This often happens if multiple Mootools or jQuery scripts are loaded.

Take a look in the sites output html (CTRL+U) to check this.

For more details about the javascript error(s), you can use Firefox with the Web Developer Toolbar, Inspect Element console in Chrome, or any other browser with a javascript debugger.

Tabs are not working on Internet Explorer (but are working on other browsers)

There are problems in the html output structure of your website

Internet Explorer is pretty picky when it comes to html. It will not handle invalid markup gracefully.

Check if you have unclosed html tags in your content (see the html view in your editor), like divs or spans that have no closing tag, or lost closing tags without opening tag.

You can check your page for html errors on:

Tabs with special characters are not working

You probably have a plugin installed that tries to optimize your html.

Extensions like 'RokBooster' and 'Content Optimizer' try to fix issues in your html. However they also mess around with the urls.

The thing that causes the issue is that these extensions url-encode special characters in urls. This is not good, seeing nowadays urls with special characters are and should be no problem.

So you will have to disable/uninstall these extensions to be able to use special characters in Tabs.

Another solution is to give the tab an alias without special characters, like:

{tab title="Ṩῥἕcịᾳḻ ḈĦẚrḯḉṯɇṛş" alias="special-characters"}

The tab title is showing in the content area

The tab title is also placed at the top of the content, but hidden. This is done for SEO purposes and to accompany the responsive view feature in the Pro version.

If you are using custom css styling, then your stylesheet probably doesn't have the css to hide the heading yet.

Simply add this css to your stylesheet:

.rl_tabs-title {
   display: none;

Will Tabs work with a Bootstrap 4 or 5 template?

Tabs (and Sliders and Tooltips) for Joomla 3 use and rely on the jQuery and Bootstrap scripts (and partially css) that comes with Joomla core.
So that is jQuery v1.12.4 and Bootstrap v2.3.2.

Bootstrap has basic tab, accordion (slider) and tooltip functionality which these Regular Labs extensions extend.

Many of the newer templates (especially the frameworks) do not use what comes with Joomla and load their own versions of jQuery and Bootstrap.

With Bootstrap 3 pretty much everything keeps working. You might need to tweak some styling.

With Bootstrap 4or 5 the javascripts and the tab and accordion functionality has changed, causing stuff based on the older versions of Bootstrap to fail.
In some cases you can 'fix' things by overriding css to force stuff to hide/display based on different classnames than the 'active' class, for instance.
But in other cases - especially when the template loads extra scripts further changing the Bootstrap functionality - it is not an easy fix.
Then you would either need to write custom javascript to over-override functionality. Or just accept that it is incompatible and look for a different solution.

There are no plans to change the reliance on Bootstrap 2 (which comes with Joomla core) for the Joomla 3 versions of Tabs, Sliders and Tooltips.

There are plans to rewrite these three javascript-based extensions entirely for Joomla 4, making them no longer reliant on either Bootstrap or jQuery.

Doing so will mean that updating Joomla 3 to Joomla 4 will probably break your customizations, as the scripts and css styling will be completely different.