ATX headers: clarify where tabs can go.

See #410.

1 files changed, 41 insertions, 23 deletions

diff --git a/spec.txt b/spec.txt
index 32c99c2..738ee96 100644
--- a/spec.txt
+++ b/spec.txt
@@ -730,17 +730,18 @@ If you want a thematic break in a list item, use a different bullet:
 
 ## ATX headings
 
-An [ATX heading](@)
-consists of a string of characters, parsed as inline content, between an
-opening sequence of 1--6 unescaped `#` characters and an optional
-closing sequence of any number of unescaped `#` characters.
-The opening sequence of `#` characters must be followed by a
-[space] or by the end of line. The optional closing sequence of `#`s must be
-preceded by a [space] and may be followed by spaces only.  The opening
-`#` character may be indented 0-3 spaces.  The raw contents of the
-heading are stripped of leading and trailing spaces before being parsed
-as inline content.  The heading level is equal to the number of `#`
-characters in the opening sequence.
+An [ATX heading](@) consists of a string of characters, parsed
+as inline content, between an opening sequence of 1--6 unescaped
+`#` characters and an optional closing sequence of any number of
+unescaped `#` characters.  The opening sequence of `#`
+characters must be followed by a [space], a tab, or by the end of
+the line. The optional closing sequence of `#`s must be preceded
+by a [space] or tab and may be followed by spaces or tabs only.
+The opening `#` character may be indented 0-3 spaces.  The raw
+contents of the heading are stripped of leading and trailing
+spaces and tabs before being parsed as inline content.  The
+heading level is equal to the number of `#` characters in the
+opening sequence.
 
 Simple headings:
 
@@ -770,7 +771,7 @@ More than six `#` characters is not a heading:
 ````````````````````````````````
 
 
-At least one space is required between the `#` characters and the
+At least one space or tab is required between the `#` characters and the
 heading's contents, unless the heading is empty.  Note that many
 implementations currently do not require the space.  However, the
 space was required by the
@@ -788,12 +789,21 @@ headings:
 ````````````````````````````````
 
 
-A tab will not work:
+A tab can be used instead of a space:
 
 ```````````````````````````````` example
 #→foo
 .
-<p>#→foo</p>
+<h1>foo</h1>
+````````````````````````````````
+
+
+Any number of spaces and tabs can be used after the `#`s:
+
+```````````````````````````````` example
+# →→  foo
+.
+<h1>foo</h1>
 ````````````````````````````````
 
 
@@ -815,16 +825,16 @@ Contents are parsed as inlines:
 ````````````````````````````````
 
 
-Leading and trailing blanks are ignored in parsing inline content:
+Leading and trailing spaces and tabs are ignored in parsing inline content:
 
 ```````````````````````````````` example
-#                  foo                     
+#  →→              foo  →→               
 .
 <h1>foo</h1>
 ````````````````````````````````
 
 
-One to three spaces indentation are allowed:
+One to three spaces indentation are allowed before the first `#`:
 
 ```````````````````````````````` example
  ### foo
@@ -878,18 +888,18 @@ It need not be the same length as the opening sequence:
 ````````````````````````````````
 
 
-Spaces are allowed after the closing sequence:
+Spaces and tabs are allowed after the closing sequence:
 
 ```````````````````````````````` example
-### foo ###     
+### foo ###→     
 .
 <h3>foo</h3>
 ````````````````````````````````
 
 
-A sequence of `#` characters with anything but [spaces] following it
-is not a closing sequence, but counts as part of the contents of the
-heading:
+A sequence of `#` characters with anything but spaces or tabs
+following it is not a closing sequence, but counts as part of
+the contents of the heading:
 
 ```````````````````````````````` example
 ### foo ### b
@@ -898,7 +908,7 @@ heading:
 ````````````````````````````````
 
 
-The closing sequence must be preceded by a space:
+The closing sequence must be preceded by at least one space or tab:
 
 ```````````````````````````````` example
 # foo#
@@ -906,6 +916,12 @@ The closing sequence must be preceded by a space:
 <h1>foo#</h1>
 ````````````````````````````````
 
+```````````````````````````````` example
+# foo→#
+.
+<h1>foo</h1>
+````````````````````````````````
+
 
 Backslash-escaped `#` characters do not count as part
 of the closing sequence:
@@ -952,10 +968,12 @@ ATX headings can be empty:
 ## 
 #
 ### ###
+####→####
 .
 <h2></h2>
 <h1></h1>
 <h3></h3>
+<h4></h4>
 ````````````````````````````````