The principal issues which have governed the decision not to permit the use of frames are:
Some visitors have graphical and text based browsers, which find it difficult and sometimes impossible to cope with frames. Framed sites can be awkward for visually-impaired visitors to interpret which can compromise accessibility.
A framed page consists of several documents. This often impedes the speedy downloading of a web page and may well alienate the visitor.
Visitors attempting to print out a specific page on a framed site may find this will end in failure.
Bookmarking pages is difficult within framed sites, as they will only be able to bookmark the URL of the home page.
If a framed site has links to other sites, the linked-to pages will load within the current frame. This may confuse visitors and will mask the URL of the destination site which makes bookmarking it problematic.
Design, Navigation and Operational Issues
Framed sites are difficult to implement well.
A web page is small to begin with. Carving it up with unnecessary frames can reduce the usable area to a tiny fraction.
Building a site around frames makes it difficult to navigate and limits you to having a very simple site. The cursor keys do not work unless you click in the frame you want to scroll, and the browsers BACK button may produce unexpected results.
If there is a problem with a framed page, it will be difficult to report since only the URL of the main page will be displayed.
Framed pages cause problems when the browser is called from another application.
The use of framesets is generally recognised to be problematic for search engines to access.
There may be copyright issues involved if another site shows up within the framed site. In any case if is unfair to the other site.
Search Engine Problems
Unexpected results can occur when a search engine indexes a framed site. Visitors who arrive on a page in a framed site from a search engine will not be entering through the site's portal and therefore will not see the frame that would normally be holding the page and associated navigational information.
Influences on this standard
UK Government Guidelines
Central Information Office guidelines - Exposing your website to search engines
"The use of framesets is generally recognised to be problematic for search engines (and certain users) to access and it tends to be increasingly avoided.....Ideally you should also provide a wholly non-frame based alternative display option"
WAI, 10 Frames
"Frames as implemented today (with the Frameset, Frame and IFrame elements) are problematic for several reasons:
Without scripting, they tend to break the 'previous page' functionality offered by browsers.
It is impossible to refer to the 'current state' of a frameset with a URI; once a frameset changes contents, the original URI no longer applies.
Opening a frame in a new browser window can disorient or simply annoy users."