asp attribute span not getting styled like plain span

Why is it that only the plain span is being styled and not the asp span? How can I fix this?

<span class="danger" asp-validation-for="SomeModel.Number"></span>
<span class="danger">some span</span>

<span class="danger" asp-validation-for="SomeModel.Number"></span>
<span class="danger">some span</span>

span.field-validation-error,
.field-validation-error,
.danger,
span {
    background: green;
}

.danger {
    color: yellow;
}

span.field-validation-error,
.field-validation-error,
.danger,
span {
    background: green;
}

.danger {
    color: yellow;
}