<style>
#size-40px {
font-size: 40px;
}
#color-red {
color: red;
}
</style>
<body>
abcd<span id="color-red">foo</span>123456<br>
abcd<span id="size-40px">foo</span>123456<br>
abcd<span id="size-40px color-red">foo</span>123456
</body>