The W3C Core Styles are style sheets that W3C made in the late 1990s and probably last updated in the late 2002.
The Core Styles is/was meant to be used by link to one the 8 style sheets "Oldstyle", "Modernist", "Midnight", "Ultramarine", "Swiss", "Chocolate", "Traditional" or "Steely" in a HTML-file:
<!DOCTYPE html>
<title>Document title</title>
<link href="https://w3.org/StyleSheets/Core/Swiss" rel="stylesheet">
<h1>Hello</h1>
<p>A <em>small</em> text sample.A more modern use would be to locally style Markdown in editors like BBEdit, iA Writer etc.
Given that the CSS and HTML spec has changed a bit since 2002 it might be worthwile to update the style sheets while at the same time follow what the W3C Core Styles page states:
- For technical or aesthetic reasons, the W3C Core Styles may change over time. However, the general look and feel of each style sheet will be preserved.