Why doesn't border of form input turn dark red?

As you can see I’m trying to make the input’s border to become dark red as a user starts using it. For some reason, this code doesn’t work.

What can be the problem? And how to fix that?

input:focus {
  border: 1px solid darkred;
}

<input type="text" name="user_input">

>Solution :

By default in several browsers there is already a style when an input is focused. Your darkred boder does work, however what you are seeing is the default style, to remove it use the following outline:none; Read more here

<style>
input:focus {
    border: 3px solid darkred;
    outline: none;
}
</style>

<input type="text" name="user_input">