onhovering over one element trigger hover in another code example

Example 1: how to change another element on hover

// If the cube is directly inside the container:
#container:hover > #cube { background-color: yellow; }

// If cube is next to (after containers closing tag) the container:
#container:hover + #cube { background-color: yellow; }

// If the cube is somewhere inside the container:
#container:hover #cube { background-color: yellow; }

// If the cube is a sibling of the container:
#container:hover ~ #cube { background-color: yellow; }

Example 2: make action on hover on other element

#container {
  width: 200px;
  height: 30px;
  border: 1px solid var(--c);
  --c:red;
}
#container:hover {
  --c:blue;
}
#container > div {
  width: 30px;
  height: 100%;
  background-color: var(--c);
}