fixed tests

Author: The-Nutty

src/AngleSharp.Css.Tests/Values/UnitConversion.cs

@@ -51,7 +51,7 @@ public void LengthToPixelsPercentThrowsOnInvalidRenderDimensions()
         public void LengthToPixelsEmThrowsOnInvalidRenderDimensions()
         {
             var l = new Length(50, Length.Unit.Em);
-            var renderDevice = new DefaultRenderDevice();
+            var renderDevice = new DefaultRenderDevice { FontSize = 0 };
             Assert.Throws<ArgumentException>(() => l.ToPixel(renderDevice));
             Assert.Throws<ArgumentException>(() => l.ToPixel(null));
         }
@@ -100,7 +100,7 @@ public void LengthToPixelsCorrectVh()
         public void LengthToPixelsCorrectVw()
         {
             var l = new Length(20, Length.Unit.Vw);
-            var renderDevice = new DefaultRenderDevice {ViewPortHeight = 1000};
+            var renderDevice = new DefaultRenderDevice {ViewPortWidth = 1000};
             Assert.AreEqual(200, l.ToPixel(renderDevice));
         }
 

src/AngleSharp.Css/Values/Primitives/Length.cs

@@ -379,15 +379,15 @@ public Double To(Unit unit, IRenderDimensions renderDimensions, Boolean isWidth
 
         private void CheckForValidRenderDimensions(IRenderDimensions renderDimensions, bool isWidth)
         {
-            if (renderDimensions == null || (isWidth ? renderDimensions.RenderWidth : renderDimensions.RenderHeight) > 0)
+            if (renderDimensions == null || (isWidth ? renderDimensions.RenderWidth : renderDimensions.RenderHeight) <= 0)
             {
                 throw new ArgumentException("A non null render device with a font size is required to calculate em or rem units.");
             }
         }
 
         private void CheckForValidRenderDimensionsForFont(IRenderDimensions renderDimensions)
         {
-            if (renderDimensions == null || renderDimensions.FontSize > 0)
+            if (renderDimensions == null || renderDimensions.FontSize <= 0)
             {
                 throw new ArgumentException("A non null render device with a font size is required to calculate em or rem units.");
             }