::first-line
is a pseudo element MDN W3Schools
p::first-line {
font-style: italic;
}
.intro {
font-weight: bold;
}
<article class="intro">
<p class="intro">First-word<br>second-word</p>
<p>First-word<br>second-word</p>
</article>
First-line [first word, first line styled bold]
second-line [second word, second line normal style]
::first-line
contains the first formatted line of an element, can only be attached to block level elements, and uses a limited set of properties. Further, though the behavior is undocumented, low specificity means lots of attached elements tend to break style.
This layout works well for multi-line comments under color-cubes, pictures of bird species, employee ID, and so on. You can setup rows of smaller specimens inside rows of fixed width/height divs floated left.
I am compiling color cubes in rows of ten. The row container div helps reduce unwanted wrap in the fluid sized cubes. But rules out wrapping cubes, forcing very small text under each cube, in some situations. You might face similar problems with unconstrained data under the first line. Styling "intro" in my sample might permit more flow control than just the ::first-line
style block.