The editor will insert small graphic elements that represent tags and keep track of their relationship. These editors will also launch browsers for you. They also let you arrange text and graphics like you would with a desktop publishing program while they work with the codes. In essence they try to do away with the need to switch to a browser to check your work. No matter which editor you use, you might wish to use the services of one of the HTML checkers dotted about the web.
Some checkers check syntax but some also comment on style issues.
Finally, once a number of pages have been created there is the task of making sure all of the links are still valid. Often documents or even servers you've refered to may disappear without warning. Programs called link verifiers exist to check for that.