Alex83,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.but{
height: 50px;
width: 100px;
padding: 0;
font-size: 12px;
}
.but span{
font-weight: bold;
}
.but:hover{
font-size: 0px;
}
.but:hover:before{
margin-top: 14px;
font-size: 14px;
content: attr(data-before);
color: rgba(255, 0, 0, 1);
}
.but:hover:after{
margin-top: 14px;
font-size: 12px;
content: attr(data-after);
font-weight: bold;
}
</style>
</head>
<body>
<button class ="but" data-before='подменный' data-after='текст'> текст<br> <span>выделенный текст</span>
</button>
</body>
</html>