Updated answer.
The region between Sin[x] and Cos[x] is Min[Sin[x], Cos[x]] <= y <= Max[Sin[x], Cos[x]]
instead of Sin[x] <= y <= Cos[x]
BTW, since $\max(f,g)-\min(f,g)=|f-g|$,that is why your integrate work.
$$ \int_a^b |f(x)-g(x)|\,\mathrm{d}x =\int_a^b \left [ \max(f(x),g(x))-\min(f(x),g(x))\right ]\,\mathrm{d}x$$
The set $$\{ (x,y) : \sin(x)\leq y\leq \cos(x) \} $$ is equivalent to $$\{ (x,y) : \sin(x) \leq y\; \text{and} \; y\leq \cos(x) \}$$ which implicitimplicitly satisfy a another condition say $\sin(x)\leq \cos(x)$
regionOne =
ImplicitRegion[Sin[x] <= y <= Cos[x], {{x, 0, 3/2 \[Pi]}, y}];
regionTwo =
ImplicitRegion[
Min[Sin[x], Cos[x]] <= y <=
Max[Sin[x], Cos[x]], {{x, 0, 3/2 \[Pi]}, y}];
{{RegionPlot[regionOne],
RegionMeasure[regionone]}, {RegionPlot[regionTwo],
RegionMeasure[regionTwo]}}
+++++++++++++++++++++++++++++++++++++++++++++++
Original answer
reg = ImplicitRegion[y <= x <= CubeRoot[y]^2 && -1 <= y <= 1, {x, y}]
RegionPlot[
DiscretizeRegion[reg, MaxCellMeasure -> {"Length" -> 0.012}]]
Integrate[1, Element[{x, y}, reg]]
Integrate[CubeRoot[y]^2 - y, {y, -1, 1}]
Or
reg = ImplicitRegion[y^3 <= x^3 <= y^2 && -1 <= y <= 1, {x, y}]
Integrate[1, Element[{x, y}, reg]]