Oftentimes, developers forget to mark up a table's header cells with the proper HTML tag. Instead, they are styled only visually as headers. But tables lacking proper header cells are tricky to navigate by screen reader users, and the more complex they are, the less they can be understood.
The table in the following example does not offer any real header cells (<th> elements); instead, only data cells are used (<td> elements). Those cells that are meant to be headers are only styled visually using CSS font-weight: bold, attached to a .th class.
<table><caption>
My Hobbies
</caption><thead><tr><tdclass="th">Name</td><tdclass="th">Description</td><tdclass="th">Additional Resources</td></tr></thead><tbody><tr><tdclass="th">Playing Soccer</td><td>
Soccer is a team sport played between two teams of eleven players with a
spherical ball.
</td><td><ahref="https://en.wikipedia.org/wiki/Association_football"
>Wikipedia</a
>
</td></tr><tr><tdclass="th">Dancing</td><td>
Dance is a performing art form consisting of purposefully selected
sequences of human movement.
</td><td><ahref="https://en.wikipedia.org/wiki/Dance">Wikipedia</a></td></tr><tr><tdclass="th">Gardening</td><td>
Gardening is the practice of growing and cultivating plants as part of
horticulture.
</td><td><ahref="https://en.wikipedia.org/wiki/Gardening">Wikipedia</a></td></tr></tbody></table>
This makes it very hard for screen reader users to keep orientation, as they do not know in which column (Name? Description? Additional Resources?) and row (Playing Soccer? Dancing? Gardening?) they are. It's especially hard in the column "Additional Resources", where the cell's content does not offer any hint on what the Wikipedia link is about.
And while this is a very small table, you may imagine how hard it will be to browse a complex table, for example with lots of columns and rows with numerical data in it.
Header cells for columns, but not for rows
The table in the following example indeed has a header cell for each column (the top row), but the data rows do not have one.
<table><caption>
My Hobbies
</caption><thead><tr><th>Name</th><th>Description</th><th>Additional Resources</th></tr></thead><tbody><tr><td>Playing Soccer</td><td>
Soccer is a team sport played between two teams of eleven players with a
spherical ball.
</td><td><ahref="https://en.wikipedia.org/wiki/Association_football"
>Wikipedia</a
>
</td></tr><tr><td>Dancing</td><td>
Dance is a performing art form consisting of purposefully selected
sequences of human movement.
</td><td><ahref="https://en.wikipedia.org/wiki/Dance">Wikipedia</a></td></tr><tr><tdclass="th">Gardening</td><td>
Gardening is the practice of growing and cultivating plants as part of
horticulture.
</td><td><ahref="https://en.wikipedia.org/wiki/Gardening">Wikipedia</a></td></tr></tbody></table>
