<span class="position-relative position-md-absolute top-md-50 top-xl-25">HELLO WORLD</span>
The "hello world" element achieves relative positioning on mobile, and absolute positioning from MD upwards. In the MD breakpoint, the top value will be 50%. In the XL breakpoint, the top will be 25%.
Resize this window to test it out: it will be below this text only on mobile devices.