Make sure line widths in the range of 0px and 1px are rounded up to 1px for visibility.

third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
  * copyright notice, this list of conditions and the following disclaimer
  * in the documentation and/or other materials provided with the
  * distribution.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 117 matching lines @@
 {
     const CSSPrimitiveValue& primitiveValue = toCSSPrimitiveValue(value);
     CSSValueID valueID = primitiveValue.getValueID();
     if (valueID == CSSValueThin)
         return 1;
     if (valueID == CSSValueMedium)
         return 3;
     if (valueID == CSSValueThick)
         return 5;
     if (valueID == CSSValueInvalid) {
-        // Any original result that was >= 1 should not be allowed to fall below 1.
-        // This keeps border lines from vanishing.
-        T result = primitiveValue.computeLength<T>(state.cssToLengthConversionData());
-        if (state.style()->effectiveZoom() < 1.0f && result < 1.0) {
-            T originalLength = primitiveValue.computeLength<T>(state.cssToLengthConversionData().copyWithAdjustedZoom(1.0));
-            if (originalLength >= 1.0)
-                return 1.0;
-        }
-        return result;
+        double result = primitiveValue.computeLength<double>(state.cssToLengthConversionData());

[eae, 2015/11/02 03:02:02]

I'd argue that a border that is specified to be 0.25 should round to zero rather
than one. Today that doesn't really matter but with the move to use full page
zoom for high-dpi 0.5 would map to 1 device pixel on high DPI. At the very least
I'd like us to clamp it to device pixels or zoom adjusted CSS pixels instead of
raw CSS pixels.

[eae, 2015/11/02 03:14:46]

It's probably more important to match the other browsers here than to match our
own legacy behavior. Let's give this a try. Please add a comment about hi-dpi
support though and reference bug 485650.

+        if (result > 0.0 && result < 1.0)
+            return 1.0;
+        return clampTo<T>(roundForImpreciseConversion<T>(result), defaultMinimumForClamp<T>(), defaultMaximumForClamp<T>());
     }
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 template <CSSValueID IdForNone>
 AtomicString StyleBuilderConverter::convertString(StyleResolverState&, const CSSValue& value)
 {
     if (value.isStringValue())
         return AtomicString(toCSSStringValue(value).value());
     ASSERT(toCSSPrimitiveValue(value).getValueID() == IdForNone);
     return nullAtom;
 }
 
 } // namespace blink
 
 #endif