Thank you in advance! Only area I was a bit iffy on was the “Add attributes to make the row and column headers unambiguously associated with the rows, columns, or rowgroups that they act as headings for.”
I decided to go with the scope attribute. It definitely was quicker but I was unsure if I should have differentiated each rowgroup for the Jovian row since there are two sub-rows (gas + ice giants).
Congratulations! Great job on this task. Your scope and *span attributes are mostly correct.
Here are some minor correction:
When using the <colgroup> element we need to make sure to cover all the columns. In your code this means to add a third <col> with an appropriate span value.
The first <tr> needs to be wrapped with <thead>
colspan="1" is the same as writing nothing. This can be removed.
“Gas giants” and “Ice giants” should have scope="rowgroup" since they span two rows
Jupiter needs scope="row"
You are missing the <tr> elements around the Saturn and Neptune rows
On a side note: You can use the W3C HTML validator to check if your code is valid.