[KevinH]

That should just be a disabled (greyed out) tool button to cycle css. It is in mine. Windows is just hiding it for some reason. Are you by chance running your own custom icon theme? If not, are you running main, material, or fluent icons? One of them must be is missing that icon.

Update: I checked and legacy, fluent, material, and main icon theme sets all have that icon and all display it both when disabled (no css styles) and when enabled. So unless you are using your own custom icon theme, there should be an icon there albeit dimmed since it is disabled. There is on my machine.

The ???????? is not a text box but instead a progress bar area. When Preview is loading a very large xhtml file, its overlay "Loading ..." will show and that progress bar will show up show progress towards completion.

On faster machines or with small xhtml files, Preview will load so fast you will not see the Overlay come up or the progress bar flash from left to right.