Users don’t see them

According to Baymard, 27% of users overlook the hidden content in the inactive tabs.

The example below from Crutchfield contains horizontal tabs.

 

Accordion

An accordion (expandable tabs) or a long expanded page with sticky tabbed navigation on scroll both perform better than horizontal tabs.

Here’s what an Accordion looks like:

 

Sticky TOC bar

And Adidas has implemented the second option, the sticky “table of contents” nav bar:

 

Both options work

So my recommendation is to go for one of those two variants mentioned above, either an accordion or a sticky navigation which auto-selects the current section as you scroll down.

EXAMPLES