NS_ASSERTION(mUnit == eCSSUnit_URL, "not a URL value"); return mValue.mURL->mString;
NS_ASSERTION(mUnit == eCSSUnit_URL || mUnit == eCSSUnit_Image, "not a URL value"); return mUnit == eCSSUnit_URL ? mValue.mURL->mString : mValue.mImage->mString;
  const PRUnichar* GetOriginalURLValue() const  {    NS_ASSERTION(mUnit == eCSSUnit_URL, "not a URL value");    return mValue.mURL->mString;  }