Semantic's name already makes clear that it pays attention to the semantics of HTML5. It is also tag-agnostic, which means you can use any HTML tags with UI elements.
In the following code, you will find a short HTML example that shows how Semantic is intended to be used:
<main class="ui three column grid"> <aside class="column">1</aside> <section class="column">2</section> <section class="column">3</section> </main>
Also, Semantic has been built with Less. The complete source, including the Less files, can be downloaded from https://github.com/semantic-org/semantic-ui/.
The way Semantic handles Less differs from Bootstrap and most of the examples you have seen earlier in this book...