fix: Handle plaintext documents with lines of 80ch (ietf-tools#5098)

Author: larseggert

ietf/static/css/document_html.scss

@@ -165,12 +165,30 @@ $tooltip-margin: inherit !default;
 }
 
 .rfcmarkup {
-
+    // A lot of plaintext documents seem to have line lengths >72ch.
+    // To handle that, we calculate with 80ch here and adjust some of the
+    // font sizes down accordingly.
     pre {
-        width: 72ch;
+        width: 80ch;
         white-space: pre-wrap;
     }
 
+    @media screen {
+        @include media-breakpoint-only(xs) {
+            font-size: min(6.75pt, var(--doc-ptsize-max));
+        }
+
+        @include media-breakpoint-up(sm) {
+            font-size: min(8.75pt, var(--doc-ptsize-max));
+        }
+
+        @include media-breakpoint-up(md) {
+            font-size: min(8.75pt, var(--doc-ptsize-max));
+        }
+
+        // Rest of font sizes is inherited from above.
+    }
+
     h1,
     h2,
     h3,