I’d like someone to help me with the table. I tried to expand the first (empty) header onto 2 columns, in order to put Jovian planets and Gas/Ice planets under one header, but it resulted into puting all planets (except Jovian ones) under this same header. Please, explain where I did a mistake
Sourse code: https://jsfiddle.net/Valdemar_check/qy1da85L/
Hi @Valdemarcheck and welcome to the community 
It’s correct to expand the empty cell (which should be a <td> 
) to two columns. Now you also have to add colspan="2" to “Terrestrial planets” and “Dwarf planets*”.
I hope that brings you further in the exercise. Please tell me if you have more questions or you want me to assess your whole work. 
Have a nice day,
Michael
Thanks for your reply, everything is fixed now!
I’m glad it was helpful! If you have updated your JSFiddle and want me to have a final look, you need to post the new link. (On JSFiddle every time you save the URL changes)