Edit line thickness of CSS 'underline' attribute

Question

Since you can underline any text in CSS like so:

h4 {
    text-decoration: underline;
}

How can you also then edit the \'line\' that is drawn

Answer 1

You can do it with a linear-gradient by setting it to be like this:

h1, a {
    display: inline;
    text-decoration: none;
    color: black;
    background-image: linear-gradient(to top, #000 12%, transparent 12%);
}

I'm underlined
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

And, yes, you can change it like this...

var m = document.getElementById("m");
m.onchange = u;
function u() {
    document.getElementById("a").innerHTML = ":root { --value: " + m.value + "%;";
}

h1, a {
    display: inline;
    text-decoration: none;
    color: black;
    background-image: linear-gradient(to top, #000 var(--value), transparent var(--value));
}

I'm underlined
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.