HTML template: remove default font size, line height...

...and font family in default inline css.  Closes #8423
(see comments there for motivation).  `mainfont`, `fontsize`,
and `linestretch` can still be used as before; the only difference
is that we no longer provide opinionated defaults.

This commit also adds a `maxwidth` variable that sets `max-width`;
if not set, 36em is used as a default.

MANUAL.txt

@@ -2641,6 +2641,9 @@ ODT or pptx.
 :   sets the CSS `line-height` property on the `html` element,
     which is preferred to be unitless.
 
+`maxwidth`
+:   sets the CSS `max-width` property (default is 32em).
+
 `backgroundcolor`
 :   sets the CSS `background-color` property on the `html` element.
 

data/templates/styles.html

@@ -1,14 +1,20 @@
 $if(document-css)$
 html {
-  line-height: $if(linestretch)$$linestretch$$else$1.5$endif$;
-  font-family: $if(mainfont)$$mainfont$$else$Georgia, serif$endif$;
-  font-size: $if(fontsize)$$fontsize$$else$20px$endif$;
+$if(mainfont)$
+  font-family: $mainfont$;
+$endif$
+$if(fontsize)$
+  font-size: $fontsize$;
+$endif$
+$if(linestretch)$
+  line-height: $linestretch$;
+$endif$
   color: $if(fontcolor)$$fontcolor$$else$#1a1a1a$endif$;
   background-color: $if(backgroundcolor)$$backgroundcolor$$else$#fdfdfd$endif$;
 }
 body {
   margin: 0 auto;
-  max-width: 36em;
+  max-width: $if(maxwidth)$$maxwidth$$else$36em$endif$;
   padding-left: $if(margin-left)$$margin-left$$else$50px$endif$;
   padding-right: $if(margin-right)$$margin-right$$else$50px$endif$;
   padding-top: $if(margin-top)$$margin-top$$else$50px$endif$;
@@ -21,7 +27,7 @@ body {
 @media (max-width: 600px) {
   body {
     font-size: 0.9em;
-    padding: 1em;
+    padding: 12px;
   }
   h1 {
     font-size: 1.8em;

test/lhs-test.html

@@ -7,9 +7,6 @@
   <title>lhs-test</title>
   <style>
     html {
-      line-height: 1.5;
-      font-family: Georgia, serif;
-      font-size: 20px;
       color: #1a1a1a;
       background-color: #fdfdfd;
     }
@@ -28,7 +25,7 @@
     @media (max-width: 600px) {
       body {
         font-size: 0.9em;
-        padding: 1em;
+        padding: 12px;
       }
       h1 {
         font-size: 1.8em;

test/lhs-test.html+lhs

@@ -7,9 +7,6 @@
   <title>lhs-test</title>
   <style>
     html {
-      line-height: 1.5;
-      font-family: Georgia, serif;
-      font-size: 20px;
       color: #1a1a1a;
       background-color: #fdfdfd;
     }
@@ -28,7 +25,7 @@
     @media (max-width: 600px) {
       body {
         font-size: 0.9em;
-        padding: 1em;
+        padding: 12px;
       }
       h1 {
         font-size: 1.8em;

test/writer.html4

@@ -10,9 +10,6 @@
   <title>Pandoc Test Suite</title>
   <style type="text/css">
     html {
-      line-height: 1.5;
-      font-family: Georgia, serif;
-      font-size: 20px;
       color: #1a1a1a;
       background-color: #fdfdfd;
     }
@@ -31,7 +28,7 @@
     @media (max-width: 600px) {
       body {
         font-size: 0.9em;
-        padding: 1em;
+        padding: 12px;
       }
       h1 {
         font-size: 1.8em;

test/writer.html5

@@ -10,9 +10,6 @@
   <title>Pandoc Test Suite</title>
   <style>
     html {
-      line-height: 1.5;
-      font-family: Georgia, serif;
-      font-size: 20px;
       color: #1a1a1a;
       background-color: #fdfdfd;
     }
@@ -31,7 +28,7 @@
     @media (max-width: 600px) {
       body {
         font-size: 0.9em;
-        padding: 1em;
+        padding: 12px;
       }
       h1 {
         font-size: 1.8em;