outline web pages

Dear Sir/Madam,

 

Last week I discovered the option "Show Outline" is no longer available
after validating a document.

Unfortunately, I used that specific option very often, to check if the
structure of my developed web pages is correct.

Can you please tell me how to check the outline of documents in the future?

 

Thank you in advance.

 

Yours sincerelly,

 

Bas Wieleman