::first-line

css/style.css

p::first-line {
color: #852E76;
background-color: #E2E8ED;
font-size: 1.2em;
}

p::first-line {
 color: #852E76;
 background-color: #E2E8ED;
 font-size: 1.2em;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<div>
<p>It was in the spring of the year 1894 that all London was
interested, and the fashionable world dismayed, by the murder of
the Honourable Ronald Adair under most unusual and inexplicable
circumstances.</p>
<p>The public has already learned those particulars
of the crime which came out in the police investigation; but a
good deal was suppressed upon that occasion, since the case for
the prosecution was so overwhelmingly strong that it was not
necessary to bring forward all the facts.</p>
</div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<div>
<p>It was in the spring of the year 1894 that all London was
interested, and the fashionable world dismayed, by the murder of
the Honourable Ronald Adair under most unusual and inexplicable
circumstances.</p>
<p>The public has already learned those particulars
of the crime which came out in the police investigation; but a
good deal was suppressed upon that occasion, since the case for
the prosecution was so overwhelmingly strong that it was not
necessary to bring forward all the facts.</p>
</div>
</body>
</html>