Petr Gazarov published a pretty rad little design pattern in his article Text input highlight, TripAdvisor style.
It’s a trick! You can’t really make an
<input> stretch like that, so Petr makes a
<span> to sync the value too, which acts as the border itself. The whole thing is a React component.
If you’re willing to use a
<span contenteditable> instead, you could do the whole thing in CSS!
See the Pen Outline bottom by Chris Coyier (@chriscoyier) on CodePen.
Although that also means no
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Text Input with Expanding Bottom Border is a post from CSS-Tricks