Today I had a question from a client: Yesterday she was working on a site, editing some texts and everything worked fine. But today, the button to add markup styles was gone! When I logged in on her machine with the admin account, thinking it maybe something to do with security, the button was still hidden. It did work on my machine though.
If you are missing some buttons on the ribbon, try zooming out. SharePoint will try to fill the ribbon place with all functions it can. When there is no more room, it will hide things.
The user was working on a flexbench, yesterday she sat in an office with a widescreen monitor, today she had a smaller screen to look at.
I decided to try it out on a virtual machine. This time the markup styles button appeared if I zoomed in 200%, but the html buttons dissapeared.
No idea how SharePoint decides what to show and what not, but if you are missing some buttons, use the zoom feature!