integral = ∫∫(Sqrt[(x^2) + 
                  (y^2)]) \[DifferentialD]x \[DifferentialD]y

XML`MathML`ExpressionToMathML[integral]

\(ExpressionToOO[x_] := 
      StringReplace[
        XML`MathML`ExpressionToMathML[
          x], {"<mo>&#8290;</mo>" -> "", "<mo>&#8289;</mo>" -> ""}]\)

ExpressionToOO[integral]