This is Markstrata - HTML itself, running in this page the way a SharePoint page runs it. The library behind it holds notes.html - a whole HTML file, with its own <style> block - and shared.css, a stylesheet several web parts could be given. Open the properties: the document is on the first page and the stylesheet on a page of its own, so one file can dress every HTML web part in a site while a document still varies with styles of its own. Try the render modes, and open the editor to see the HTML and the CSS as two tabs over one preview. The cream behind the document is that file’s own body rule, and it stops where the document stops: an author styles their document, not the toolbar and the footer around it.
This paragraph is outside the web part and has no styling of its own, so anything that colours it is a rule that escaped the document. A link outside the web part, for the same reason.
