This is the title of Chapter 1 in the new book by Dan Cederholm, Handcrafted CSS. I've been dealing with this a lot lately myself and have been doing much thinking regarding the idea of "designing in a vacuum". I love this book! Handcrafted CSS is full of Web design gold. Dan goes on to say:

"Often the decisions we make as designers and design implementers directly reflect the integrity of the interface. What happens if there is more (or less) content on the page than planned? What happens if the text size of the page increases or decreases? What happens if there are two paragraphs instead of one? And what if one of those paragaphs is translated into, say, German?

Flexibility in Web design is the mark of a true craftsman. Designing something statically in an image editor is one achievement, but allowing for give and take within the boundaries of a page layout is what sets good Web design apart from great Web design. It's taking the Web as a medium into account as part of the design process, and letting go of pixel precision."

One of my old bosses and design mentors earlier in my career used to say this exact same thing all the time. "What if this changes" was a question he was always asking me, challenging me to ensure that what I was designing was flexible. I'm thankful for that early mentoring. If you're a designer or front-end developer, do yourself a favor and grab a copy of this book.