The most basic and essential of all tests is a check against original designs. Of course, it goes without saying that the original designs should be a precise as humanely possible, without deviations and errors. The developed extension should then fall within a certain proximity to those designs. Note that we should give some leeway and be comfortable with some delta between the designs and what's been developed. But fundamentally – the closer the better.
One good tool recommended for checking what's been developed against the designs is xScope (Mac tool).
xScope enables live checking of distances (paddings and margins), colours visible on screen, pixel measurements, and similar visible details. Here's an example of xScope in action measuring the size of one tile in Guided Development (400px), checking colours, and using red lines to inspect height and width of the third tile.