The string appears on all <body> tags in some old asp.net webform applications.
If i remember right it was used by Visual Studio early on to say whether you wanted to view the design view in Grid or Flow layout mode. I don't think it is used in more current versions and it should have no effect on the browser.
VS.NET uses this property to define whether the page (or other container such as Panel control) uses absolute positioning (GridLayout) or not.
I prefer FlowLayout because it works in all browsers AND because it prevents controls from shifting on the page during browser resizes.
In general you should not use grid layout as it does not support browsers with different screen reslutions or font sizes than the one designed on.
They mean nothing to the browser. they are just hints to editing tools. (Visual Studio-Specific)