Make the element to be inline-block
and it won’t get affected by the underline:
a {
text-decoration: underline;
}
a #myspan {
color: black;
display:inline-block;
}
a:active #myspan {
color: grey;
}
a:visited #myspan {
color: yellow;
}
a:hover #myspan {
color: red;
}
<a href="#">A link <span id="myspan">some additional information</span></a>
Note that text decorations are not propagated to floating and absolutely positioned descendants, nor to the contents of atomic inline-level descendants such as inline blocks and inline tables. ref
To remove the small space between text and span you can get rid of the whitespace and use a small margin:
a {
text-decoration: underline;
}
a #myspan {
color: black;
display:inline-block;
margin-left:4px;
}
a:active #myspan {
color: grey;
}
a:visited #myspan {
color: yellow;
}
a:hover #myspan {
color: red;
}
<a href="#">A link<span id="myspan">some additional information</span></a>