Tooltip

Move the mouser over the main element to see the tooltip.
<div class="container">
<!-- The tooltip content -->
<div class="container__content">
...
</div>
<!-- The tooltip arrow -->
<div class="container__arrow" />
<!-- The trigger element -->
...
</div>
.container {
/* Used to position the arrow */
position: relative;
}
/* Show the arrow and content and restore pointer events when hovering the trigger element */
.container:hover .container__arrow,
.container:hover .container__content {
opacity: 1;
pointer-events: initial;
}
.container__arrow {
/* Invisible by default */
opacity: 0;
/* To prevent accidental interactions with other elements */
pointer-events: none;
/* Border */
border: 8px solid transparent;
border-top-color: #00439e;
/* Position */
bottom: 100%;
left: 50%;
position: absolute;
transform: translate(-50%, 8px);
/* Zero size */
height: 0;
width: 0;
/* Displayed on top of other element */
z-index: 10;
}
.container__content {
/* Invisible by default */
opacity: 0;
/* To prevent accidental interactions with other elements */
pointer-events: none;
/* Background color, must be the same as the border color of arrow */
background-color: #00439e;
border-radius: 2px;
/* Position */
bottom: 100%;
left: 50%;
position: absolute;
transform: translate(-50%, -8px);
/* Displayed on top of other element */
z-index: 10;
}

See also

Popover arrow