WIP: Emit semantic HTML instead of blockquotes for notes

book.toml

@@ -3,6 +3,8 @@ title = "The Rust Programming Language"
 authors = ["Steve Klabnik", "Carol Nichols", "Contributions from the Rust Community"]
 
 [output.html]
-additional-css = ["ferris.css", "theme/2018-edition.css"]
+additional-css = ["ferris.css", "theme/2018-edition.css", "theme/semantic-notes.css"]
 additional-js = ["ferris.js"]
 git-repository-url = "https://github.com/rust-lang/book"
+
+[preprocessor.trpl-note]

theme/semantic-notes.css

@@ -0,0 +1,13 @@
+/*
+  This is copied directly from the styles for blockquotes, because notes were
+  historically rendered *as* blockquotes. This keeps the presentation of them
+  identical while updating the presentation.
+*/
+.note {
+    margin: 20px 0;
+    padding: 0 20px;
+    color: var(--fg);
+    background-color: var(--quote-bg);
+    border-block-start: 0.1em solid var(--quote-border);
+    border-block-end: 0.1em solid var(--quote-border);
+}