IntroToWebAuthoring/pages/main.xml

<ConstraintLayout>
    <BoxView margin="10" align_top_to_top="parent" align_left_to_left="parent">
            <ColoredRectView r="123" g="12" b="34" />
    </BoxView>

    <BoxView
        margin_left="10"
        margin_right="10"
        align_top_to_top="parent"
        align_left_to_left="parent"
        align_bottom_to_bottom="parent"
        align_right_to_right="parent"
        width="400"
        height="parent"
    >
            <ColoredRectView r="30" g="30" b="30" />
    </BoxView>

    <BoxView
        margin="10"
        align_top_to_top="parent"
        align_left_to_right="2"
        align_bottom_to_bottom="parent"
        width="200"
    >
            <ColoredRectView r="123" g="125" b="34" />
    </BoxView>
</ConstraintLayout>