HTML can represent many complicated structures such as inline DIVs or SPAN’s side by side. HTML elements overlapping and borders applied to different HTML elements. For the most part this wouldn’t be a sensible approach in DOCX while it would be possible to create floating HTML elements with text boxes it would result in almost all content being contained within text boxes resulting in a very ugly and messy Word document.
It is because of this issue we ignore the floating of HTML elements and borders of most HTML elements. However we do respect borders on some HTML elements like table cells and alignment on image elements for example.
Does this mean you can't place content side-by-side? No. This is still possible by using columns, tables. There is also tab stops for content aligned inside a DIV.