We're building a web app, and the technology we're using will only allow users with relatively modern browsers (IE8 and up, plus most versions of other browsers) to even log in. We want to show an "unsupported browser"-warning when a user tries to access the site with an older browser.
What are some best practices when designing and showing such a warning? For example, should it take over the entire page and not allow the user to take any actions at all, or should it allow the user to attempt to use the app even though a lot of things will be broken? What information should be given in the warning message, etc.?