Tighten color and fieldset defaults

Use `Math.Clamp` for channel bounds in `ColorParser` and `TryAdd` the default fieldset border in `HtmlParser` so existing borders are preserved.

Author: SimonCropp

src/OpenXmlHtml/ColorParser.cs

@@ -270,7 +270,7 @@ static bool TryParseComponent(ReadOnlySpan<char> value, out int component)
     }
 
     static int Clamp(int value) =>
-        Math.Max(0, Math.Min(255, value));
+        Math.Clamp(value, 0, 255);
 
     static bool IsValidHex(ReadOnlySpan<char> hex)
     {

src/OpenXmlHtml/HtmlParser.cs

@@ -322,10 +322,7 @@ internal static void ApplyElementFormatting(IElement element, string tag, ref Fo
         if (tag == "fieldset")
         {
             declarations ??= new(StringComparer.OrdinalIgnoreCase);
-            if (!declarations.ContainsKey("border"))
-            {
-                declarations["border"] = "1pt solid #808080";
-            }
+            declarations.TryAdd("border", "1pt solid #808080");
         }
     }