The Experimentals include experimental elements and features, mostly are not yet ready for wide use. These elements and components are limited inside spectre-exp.less before browsers fully support them. Sometimes, there are some specific browsers targeted components.
The Progress indicates the progress completion of a task.
<progress class="progress" value="25" max="100"></progress>
Sliders are for selecting values from ranges.
<input class="slider" type="range" min="0" max="100" value="50" />