Hi there,
the H1 would have to be inline and the border would need to be on a pseudo element:
eg.
h1 {
position: relative;
display: inline;
}
h1:after {
content: '';
position: absolute;
bottom: -10px;
left: 0;
width: 100%;
border-bottom: 10px solid;
}
The bottom -10px is what offsets the border position.
The downside is that inline elements won’t attract top/bottom spacing, so you will need to apply that to the H1s parent container or elements that come after it