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Disallow negative text indent
Negative text indents are typically used to hide text off-screen for accessibility purposes. Often used as part of an image replacement technique, using a negative text indent ensures that screen readers will read the text even though it appears offscreen. Using visiblity: hidden or display: none causes the text to be skipped by screen readers, so a negative text indent has been deemed as better for accessibility.
The technique usually involves a very large negative number such as -999px or -9999px, such as:
.mybox {
background: url(bg.png) no-repeat;
text-indent: -9999px;
}The intent of this technique is to allow the background image to show through for sighted users while screen readers receive the inline text instead.
Negative text indents are also problematic when used in a right-to-left language page, as the effect may cause a long horizontal scrollbar to appear. This problem can be fixed by adding direction: ltr to the rule, such as:
.mybox {
background: url(bg.png) no-repeat;
direction: ltr;
text-indent: -9999px;
}There are mixed opinions about whether or not negative text indents affect a page's search ranking. Anecdotal accounts seems to indicate Google treats negative text indents as a spam technique, but this has not been confirmed.
Rule ID: text-indent
This rule is intended to find problematic uses of text-indent in CSS code. It warns when a text-indent value of -99 or smaller (i.e., -100, -999, etc.) is used without the use of direction: ltr. The units being used don't matter, so px, em, etc. are all treated as the same.
The following patterns are considered warnings:
/* missing direction */
.mybox {
text-indent: -999px;
}
/* missing direction */
.mybox {
text-indent: -999em;
}
/* direction is rtl */
.mybox {
direction: rtl;
text-indent: -999em;
}The following patterns are considered okay and do not cause a warning:
/* direction used */
.mybox {
direction: ltr;
text-indent: -999em;
}
/* Not obviously used to hide text */
.mybox {
text-indent: -1em;
}