How to show Modal and Modeless dialog windows in Javascript?

When you show a modal dialog the window remains on top of other windows until the user explicitly closes it.

window.showModalDialog('Test.html','dialogWidth:400px; dialogHeight:225px; status:no; center:yes');

When you show a modeless dialog the window remains on top of other windows, but you can still access the other windows.

window.showModelessDialog('Test.html','dialogWidth:400px; dialogHeight:225px; status:no; center:yes');