What does the "~" (tilde/squiggle/twiddle) CSS selector mean?

Good to also check the other combinators in the family and to get back to what is this specific one.

• ul li
• ul > li
• ul + ul
• ul ~ ul

Example checklist:

• ul li - Looking inside - Selects all the li elements placed (anywhere) inside the ul; Descendant combinator
• ul > li - Looking inside - Selects only the direct li elements of ul; i.e. it will only select direct children li of ul; Child combinator
• ul + ul - Looking outside - Selects the ul immediately following the ul; It is not looking inside, but looking outside for the immediately following element; Adjacent sibling combinator / Next-sibling combinator
• ul ~ ul - Looking outside - Selects all the following ul's, but both ul's should be having the same parent; General sibling combinator / Subsequent-sibling combinator

The one we are looking at here is the General sibling combinator / Subsequent-sibling combinator

The ~ selector is in fact the subsequent-sibling combinator (previously called general sibling combinator until 2017):

The subsequent-sibling combinator is made of the "tilde" (U+007E, ~) character that separates two sequences of simple selectors. The elements represented by the two sequences share the same parent in the document tree and the element represented by the first sequence precedes (not necessarily immediately) the element represented by the second one.

Consider the following example:

.a ~ .b {
  background-color: powderblue;
}

<ul>
  <li class="b">1st</li>
  <li class="a">2nd</li>
  <li>3rd</li>
  <li class="b">4th</li>
  <li class="b">5th</li>
</ul>

.a ~ .b matches the 4th and 5th list item because they:

• Are .b elements
• Are siblings of .a
• Appear after .a in HTML source order.

Likewise, .check:checked ~ .content matches all .content elements that are siblings of .check:checked and appear after it.

General sibling combinator

The general sibling combinator selector is very similar to the adjacent sibling combinator selector. The difference is that the element being selected doesn't need to immediately succeed the first element, but can appear anywhere after it.

More info