For default browser settings, the PX & EM values will be the same, and background colors will change at the same time when you resize from ~desktop to ~mobile size
But compare layout across changing browser zoom (eg. `cmd+`) vs increasing font size via browser settings (found under "advanced" in Chrome, Preferences > Content in FF) - the EM and the PX-based queries trigger at different sizes
Additionally, Safari 8 shows the same inconsistent behavior for both `cmd+` and text zooming