diff --git a/include/bx/inline/math.inl b/include/bx/inline/math.inl
index 649ef93..3b39d79 100644
--- a/include/bx/inline/math.inl
+++ b/include/bx/inline/math.inl
@@ -259,7 +259,8 @@ namespace bx
 
 	inline BX_CONST_FUNC bool equal(float _a, float _b, float _epsilon)
 	{
-		// http://realtimecollisiondetection.net/blog/?p=89
+		// Reference:
+		// https://web.archive.org/web/20181103180318/http://realtimecollisiondetection.net/blog/?p=89
 		const float lhs = abs(_a - _b);
 		const float rhs = _epsilon * max(1.0f, abs(_a), abs(_b) );
 		return lhs <= rhs;