Design features the following components:
- Grid - overlays a highly configurable layout grid over a web-page. Grid can be set to match any set of dimensions, allowing easy development of CSS layouts in the web-browser.
Rule - displays rulers on a page, with all the expected features of the rulers found in a desktop design application, including guides which snap to block display elements and origin location control.
Unit - allows measurements to be made between any two points on a web-page, giving basic information about each of the points clicked on, and drawing a line on the page for the measurement.
Crosshair - draws a crosshair cursor on the page to assist in layout alignment. Cursor information is also presented in a tooltip.
Link: http://sprymedia.co.uk/article/Design