Make @emph nestable, logical markup

scribble-doc/scribblings/scribble/base.scrbl

@@ -467,7 +467,9 @@ gets progressively smaller.}
 gets progressively larger.}
 
 @defproc[(emph [pre-content pre-content?] ...) element?]{
-The same as @racket[italic].}
+Like @racket[elem], but emphasised. Typically, italics are used for
+emphasis. Uses of @racket[emph] can be nested; typically this causes
+the text to alternate between italic and upright.}
 
 @defproc[(literal [str string?] ...+) element?]{
 

scribble-lib/scribble/base.rkt

@@ -258,7 +258,7 @@
   (make-element 'larger (decode-content str)))
 
 (define (emph . str)
-  (make-element 'italic (decode-content str)))
+  (make-element 'emph (decode-content str)))
 
 (define (tt . str)
   (let* ([l (decode-content str)]

scribble-lib/scribble/html-render.rkt

@@ -1607,6 +1607,7 @@
        (cond
         [(symbol? name)
          (case name
+           [(emph) '([class "emph"])]
            [(italic) '([style "font-style: italic"])]
            [(bold) '([style "font-weight: bold"])]
            [(tt) '([class "stt"])]

scribble-lib/scribble/latex-render.rkt

@@ -471,6 +471,7 @@
               (cond
                [(symbol? style-name)
                 (case style-name
+                  [(emph) (wrap e "emph" tt?)]
                   [(italic) (wrap e "textit" tt?)]
                   [(bold) (wrap e "textbf" tt?)]
                   [(tt) (wrap e "Scribtexttt" #t)]

scribble-lib/scribble/markdown-render.rkt

@@ -216,6 +216,9 @@
     (define (italic? i)
       (and (element? i) (eq? (element-style i) 'italic)))
 
+    (define (emph? i)
+      (and (element? i) (eq? (element-style i) 'emph)))
+
     (define (code? i)
       (and (element? i)
            (let ([s (element-style i)])
@@ -259,6 +262,12 @@
         [(and (italic? i) (not (in-italic?)) (not (in-code?)))
           (recurse-wrapped "_" in-italic?)]
 
+        [(and (emph? i) (not (in-code?)))
+         (display "​_") ;; zero-width space, underscore
+         (begin0
+             (super render-content i part ri)
+           (display "_​"))] ;; underscore, zero-width space
+
         [(and (preserve-spaces? i) (not (preserving-spaces?)))
           (parameterize ([preserving-spaces? #t])
             (render-content i part ri))]
@@ -378,4 +387,3 @@
             ([ptn (map car ptns&reps)]
              [rep (map cdr ptns&reps)])
     (regexp-replace* ptn str rep)))
-

scribble-lib/scribble/scribble.css

@@ -30,6 +30,26 @@
   font-weight: bold;
 }
 
+/* Emphasis: alternate italics and normal as we nest */
+.emph {
+  font-style: italic;
+}
+.emph .emph {
+  font-style: normal;
+}
+.emph .emph .emph {
+  font-style: italic;
+}
+.emph .emph .emph .emph {
+  font-style: normal;
+}
+.emph .emph .emph .emph .emph {
+  font-style: italic;
+}
+.emph .emph .emph .emph .emph .emph {
+  font-style: normal;
+}
+
 /* ---------------------------------------- */
 
 p, .SIntrapara {