See http://wpftutorial.net/DrawOnPhysicalDevicePixels.html for original article.
WPF is resolution-independent. WPF logical units are 1/96 inches (so width=96 means the UI element has a width of 1 inch).
When the screen has the resolution of 96 dpi one WPF logical unit matches one pixel. Once the screen resolution changes, this rule no longer applies.
When implementing user controls the developer can use the SnapToDevicePixels property to ensure the control boundaries are not blurry.
If needed, use GuidelineSets.