‘Belt-and-suspenders’ describes an approach to web design where provision has been made for more than one method of achieving a satisfactory outcome. If one method fails (belt) then decorum is maintained by another (suspenders).
This phrase is commonly used in reference to issues of cross-browser compatibility. A feature supported by one browser may not be supported by another. For example: if a webpage uses frames, providing no-frames content that links to a sitemap (not reliant on frames) will ensure those using older browsers are still able to navigate the website.