The opacity() CSS function applies transparency to the samples in the input image. Its result is a <filter-function>.
Note: This function is similar to the more established opacity property. The difference is that with filters, some browsers provide hardware acceleration for better performance.
opacity(amount)
amount<number> or a <percentage>. A value of 0% is completely transparent, while a value of 100% leaves the input unchanged. Values between 0% and 100% are linear multipliers on the effect. The lacuna value for interpolation is 1.opacity(0%) /* Completely transparent */ opacity(50%) /* 50% transparent */ opacity(1) /* No effect */
<filter-function>opacity property
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https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/opacity