Inform the user before changing focus from the current window or before causing a "pop-up" or other new window to appear.
Example One
In the case you need to provide a pop-up window, be sure to label the link to and from the pop-up window. The link below is one example:
Click here for a list of summer classes (opens new window).
The HTML
<a href="smallwindow.htm" target="_blank">Click here for a list of summer classes (opens new window)</a>.;
HTML for pop-up window
<a href="priority2_7.htm" target="_parent">Close window to return to Priority 2 Item 7</a>
Example Two
Javascript can be used to create a pop-up window; however, some users may have javascripts turned off.
Click here for a list of summer classes (opens new window).
The HTML
<a href="javascript:void(window.open
('smallwindow.htm','VoiceImages','menubar
,scrollbars,resizable,width=500,height=200'))
">Click here for a list of summer
classes (opens new window)</a>.
HTML for pop-up window
<a href="priority2_7.htm" target="_parent">Close window to return to Priority 2 Item 7</a>
Example Three
You can also use the title tag to inform the user of a change in focus.
Click here for a list of summer classes.
The HTML
<a href="smallwindow.htm" target="_blank" title="List of summer classes (opens a new window).">Click here for a list of summer classes.</a>
HTML for pop-up window
<a href="priority2_7.htm" target="_parent">Close window to return to Priority 2 Item 7</a>
Pop-ups may cause problems for screen reader users because they take the focus without warning. This means that the screen reader begins reading the content of the new window, usually without notifying the user that a new window has opened. If the new window contains content from another site, the user can become very disoriented and confused.
View WAI checkpoint 10.1 - Until user agents allow users to turn off spawned windows, do not cause pop-ups or other windows to appear and do not change the current window without informing the user.