JSFiddle - React, Tailwind, and code Playground

HTML

<div id=page>
  <a id=ahover href="#">Hover me</a><br>
  Here is the box that slides:
  <div id=slidebox>
    I am the content of the slide box (line1).<br>
    I am the content of the slide box (line2).<br>
    <a id=button href="#">I am some button in the slide box</a><br>
    I am the content of the slide box (line3).<br>
    I am the content of the slide box (line4).
    <div id=someform>
      Some box with a form or anything...<br>
      <input type=text value="Text Box">
    </div>
    I am the content of the slide box (line5).<br>
    I am the content of the slide box (line6).
  </div>
  And this is after the box.
</div>

CSS

#page {
  font-size: calc( (35vw + 65vh) / 30); /* just some responsive design as a bonus */
}
#slidebox {
  background-color: #e8e8e8;
  visibility: hidden;
  font-size: 0; /* animated from 0 to 100% */
  opacity: 0; /* optional */
  transition: 0.5s;
}
a#ahover:hover ~ #slidebox {
  visibility: visible;
  font-size: 100%; /* animated from 0 to 100% */
  opacity: 1; /* optional */
}
a#ahover:not(:hover) ~ #slidebox * {
  border-top: 0;
  border-bottom: 0;
  /* Put here anything vertical that uses px as unit, in this case the borders */
}
a#button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  position: relative;
  width: 20em;
  height: 3em;
  padding: 0.5em;
  border: 2px solid #0080ff;
  border-radius: 0.4em;
  background-color: #8fbfef;
  color: #404040;
  box-sizing: border-box;
}
#someform {
  margin-top: 1em;
  margin-bottom: 1em;
  padding: 1em 4em 1em 4em;
  background-color: #d8ffd8;
  border: 1px solid #888888;
}
#someform input {
  display: inline-block;
  box-sizing: border-box;
  font-size: 125%;
  padding: 0.5em;
  width: 50%; /* anything horizontal can still be in px or % */
  border-radius: 0.4em;
  border: 1px solid red;
}