pxt-core/react-common/styles/react-common-accessibility.less

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// Hide content but keep component around for screen readers
.sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    border: 0;
    white-space: nowrap;
    border-width: 0;
}