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