Have you ever found yourself browsing the web on a smartphone and clicked on something that opened in a new tab? It is almost impossible to tell when this happens. In fact, many people don't even realize that browsers on SmartPhones can have the equivalent of multiple tabs or windows.
Now imagine someone using a screen magnifier. They click on a link somewhere in the middle of the screen. If the link opened in a new tab in the background, they would never notice it and just think something is broken. However, if it opened in the foreground, one might think everything is good. Except that they will not realize that they are now looking at content in a new tab. So when they click BACK to return to the previous page, it simply won't work since there is no previous page in that tab.
As you can tell, these are good reasons to avoid opening content in a new browser tab. However, if you really want to open content in a new tab, you can make it accessible and friendly by simply indicating that the link will open in a new tab. Example: Google (will open in a new tab). Do make the words part of the link so that screen readers will pick it up too as users tab through the links on a page.
Reminder: Most links can be manually opened in a new tab by the user in several ways. If they are aware of how to use tabs, why not leave it in their control? If they don't know how to use tabs, why impose it on them, lead them into confusion and leave them disorientated?
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