You shouldn't put the <html> tag anywhere in your story, Twine takes care of that for you.
The Stylesheet section is just for CSS. For example:
html {
font: 16px/1 'Meiryo', 'Segoe UI', 'Tahoma', 'Arimo', sans-serif;
width: 100%; /* IE full screen fix */
}
#story-author {
display: none;
}
#ui-bar-body>:not(:first-child) {
margin-top: 1.5em;
}
div#story-caption {
margin-top: 0.5em !important;
}
table {
margin: 0px !important;
width: 100%;
}
td {
line-height: 1.4em;
}
input {
padding: 2px;
}
button {
padding: 2px .4em;
}
#story {
margin-right: 30px;
}
For more information on how to use CSS see here:
https://developer.mozilla.org/en-US/docs/Web/CSS
https://www.w3schools.com/css/default.asp