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outline is your friend

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If you open a plain HTML document with no CSS and you focus an interactive element like a button, link, or textarea, you’ll see that by default browsers use the outline property to highlight these elements.

A simple demo of a link a button and a textarea

A blue outline around a focused button in Firefox

outline is great

The outline property is the perfect candidate for this job for several reasons.

outline sucks

There are also disadvantages to using outline.

What about other properties?

If you remember, one of the advantages of outline is that it works great in forced color mode. If this property works well, it means that there must be properties that don’t work well.

Here’s an example. Instead of using outline, I change the background and add a box-shadow on focus-visible.

:focus:not(:focus-visible) {
outline: none;
}

:focus-visible {
outline: none;
background: #000;
color: #fff;
box-shadow: 0 2px 0 0 #fff, 0 5px 0 0 #000;
}

This is how focus styles on the button look like with regular contrast settings.

Dark background color on the button and a thick dark line below

And here’s the same focused button in WHCM. We don’t see anything because these properties are reverted in forced color modes.

The styling of the button didn’t change at all

Transparent outlines

Now you might be thinking “But…but a11yproject.com and gov.uk are using background colors for some of their focus styles”. Yes, they are using background-color, but they’re doing it in combination with outline. This is a nice trick to avoid visible outlines in normal color mode and assure that they’re displayed in forced color mode.

.c-homepage-card__cta:focus {
background-color: #fb37ff;
color: #000;
outline: 3px solid #fb37ff;
}
pink background on focused links.

a11yproject.com uses the same color for the outline as for the background.

just an outline on focused links.
No background color in WHCM.
.govuk-link:focus {
outline: 3px solid transparent;
color: #0b0c0c;
background-color: #fd0;
box-shadow: 0 -2px #fd0,0 4px #0b0c0c;
text-decoration: none;
}
yellow background an think black underline on focused links

gov.uk uses a background color and box-shadow in combination with a transparent outline

just an outline on focused links.

No background color and box shadow in WHCM.

Conclusion

Be careful when customizing focus styles, don’t forget to test in forced color modes, and always remember that outline is your friend.