[html] How to change border color of textarea on :focus

You can use CSS's pseudo-class to do that. A pseudo-class is used to define a special state of an element.

there is a ::focus pseudo-class that is used to select the element that has focus. So you can hook it in your CSS like this

Using class

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.my-input::focus {
  outline-color: green;
}
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Using Id

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#my-input::focus {
  outline-color: red;
}
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Directly selecting element

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input::focus {
  outline-color: blue;
}
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Using attribute selector

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input[type="text"]::focus {
  outline-color: orange;
}
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