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HTML/CSSIntermediate#css#bem
HTML/CSSAdvanced#css#stacking-context
What creates a new stacking context in CSS?
Besides position + non-auto z-index, a new stacking context is created by: opacity less than 1, transform/filter/perspective other than none, will-change, position: fixed/sticky, and flex/grid items with z-index set.
Example
.card {
opacity: 0.99; /* creates a new stacking context */
transform: translateZ(0); /* also creates one */
}