hover actions with multiple divs

Question

Is there a way to trigger an action on a div, if the hover occurred on another one, and the divs are not inside each other?

This only seems to work for me if the divs are inside each other, otherwise nothing happens.

.blockA {
  width: 64px;
  height: 64px;
  background-color: red;
}

.blockB {
  width: 64px;
  height: 64px;
  background-color: orange;
  transition: 0.5s ease-out 100ms;
}

.blockA:hover .blockB {
  background-color: blue;
}

<div class="blockA"></div>
<div class="blockB"></div>

Answer 1

You can only do this if the element you want affected is preceded by the hovered element. In this case, if you want element .blockB to be affected by the hover of .blockA, you can use the general ~ or next element + selector. However, if you want .blockA to be affected by the hover on .blockB you need to use Javascript, since there is not a preceding element selector in CSS

.blockA {
    width: 64px;
    height: 64px;
    background-color: red;
}

.blockB {
    width: 64px;
    height: 64px;
    background-color: orange;
    transition: 0.5s ease-out 100ms;
}

/* General Sibling Selector */
.blockA:hover ~ .blockB {
    background-color: blue;
}

.blockC {
    width: 64px;
    height: 64px;
    background-color: red;
}

.blockD {
    width: 64px;
    height: 64px;
    background-color: orange;
    transition: 0.5s ease-out 100ms;
}

/* Next element selector */
.blockC:hover + .blockD {
    background-color: blue;
}

<div class="blockA"></div>
<div class="blockB"></div>

<div class="blockC"></div>
<div class="blockD"></div>

If you want to also affect the previous element on hover, here is a snippet with the JS to handle that. We would need to add a class to the second .second element to know what preceding element to affect on hover.

// Get all the elements with the class "second"
let second_elem = document.querySelectorAll('.second');

// This loops through each second_elem
second_elem.forEach(elem => {
  // We are now adding an event listener - in this case when the mouse enters the element.
  elem.addEventListener('mouseenter', (event) => {
    // The event target is the element that is hovered. We get the immediate preceding element.
    let prev_elem = event.target.previousElementSibling;
    // Add the "hovered" class. In this case to blockA.
    prev_elem.classList.add('hovered');
  });
  // Adds an event listener for when the mouse leaves the second element
  elem.addEventListener('mouseleave', (event) => {
    let prev_elem = event.target.previousElementSibling;
    // remove the "hovered" class. In this case from blockA
    prev_elem.classList.remove('hovered');
  });
});

/* We can also make the JS event listeners into a function, but for this case, just keeping it simple. */

.blockA {
  width: 64px;
  height: 64px;
  background-color: red;
  transition: 0.5s ease-out 100ms;
}

.blockB {
  width: 64px;
  height: 64px;
  background-color: orange;
  transition: 0.5s ease-out 100ms;
}


/* Next element selector */

.blockA:hover+.blockB {
  background-color: blue;
}


/* when the hovered class gets added by the JS */

.blockA.hovered {
  background-color: green;
}

<div class="blockA"></div>
<div class="blockB second"></div>

Answer 2

Add a comma between .blockA:hover .blockB like so:

.blockA:hover, .blockB {
    background-color: blue;
}

<div class="blockA">Put something here</div>
<div class="blockB">Put something here</div>

Also, I think you want to use .blockB:hover in that case. What happens when there's no comma is that it checks when .blockA is hovered and if so it applies background-color: blue to all elements inside it that have .blockB