urbit/outside/commonmark/alternative-html-blocks.txt

# Appendix B: An alternate spec for HTML blocks {-}

(The following spec departs less from original markdown than the
one described above, but is also less flexible.)

An [HTML block](#html-block) <a id="html-block-tag"/> begins
with an [open tag](#open-tag), [HTML comment](#html-comment),
[processing instruction](#processing-instruction),
[declaration](#declaration), or [CDATA section](#cdata-section).
This opening element may optionally be preceded by 1-3 spaces,
and must not be followed on a line by anything other than white space.

If the opening tag is self-closing, or if it is an [HTML
comment](#html-comment), [processing
instruction](#processing-instruction), [declaration](#declaration), or
[CDATA section](#cdata-section), then the [HTML block](#html-block)
contains just that tag.

If it is an [open tag](#open-tag), then the [HTML block](#html-block)
continues until a matching closing tag is found, or until the end
of the document.  Note that the matching closing tag is not necessarily
the first closing tag of the same type that is encountered, since
that tag may close a later open tag of the same type.  Open and closing
tags must be balanced.

The contents of the HTML block are interpreted as raw HTML, and will not
be escaped in HTML output.

Some simple examples:

.
<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>

okay.
.
<table>
  <tr>
    <td>
           hi
    </td>
  </tr>
</table>
<p>okay.</p>
.


.
<div class="outer">

    <div class="inner">

      <p>foo&ouml;</p>

    </div>

</div>
.
<div class="outer">

    <div class="inner">

      <p>foo&ouml;</p>

    </div>

</div>
.

A self-closing tag:

.
<div />
.
<div />
.

Here we have an unclosed tag, and the block continues to the end of
the document:

.
<div>
<div>
foo
</div>

*bar*
.
<div>
<div>
foo
</div>

*bar*
.

A comment:

.
<!-- Foo
bar
   baz -->
.
<!-- Foo
bar
   baz -->
.

A processing instruction:

.
<?php
  echo 'foo'
?>
.
<?php
  echo 'foo'
?>
.

CDATA:

.
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then
  {
  return 1;
  }
else
  {
  return 0;
  }
}
]]>
.
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then
  {
  return 1;
  }
else
  {
  return 0;
  }
}
]]>
.

The opening tag can be indented 1-3 spaces, but not 4:

.
  <!-- foo -->
    <!-- foo -->
.
  <!-- foo -->
<pre><code>&lt;!-- foo --&gt;
</code></pre>
.

The opening tag must be on a line (or lines) by itself:

.
<table><tr><td>
foo
</td></tr></table>
.
<p><table><tr<td> foo </td></tr></table></p>
.

.
<!-- foo -->bar
.
<p><!-- foo -->bar</p>
.

The opening tag need not be an HTML block tag or even an HTML tag:

.
<a>
foo
</a>
.
<a>
foo
</a>
.

.
<foo>
bar
</foo>
.
<foo>
bar
</foo>
.

So, note the difference:

.
<del>
bar
</del>

<del>bar</del>
.
<del>
bar
</del>
<p><del>bar</del></p>
.

This rule differs from John Gruber's original markdown syntax
specification, which says:

> The only restrictions are that block-level HTML elements —
> e.g. `<div>`, `<table>`, `<pre>`, `<p>`, etc. — must be separated from
> surrounding content by blank lines, and the start and end tags of the
> block should not be indented with tabs or spaces.

In some ways Gruber's rule is more restrictive than the one given
here:

- It requires that an HTML block be preceded and followed by a blank line.
- It does not allow the start tag to be indented.
- It does not allow the end tag to be indented.
- It does not require that the open tag be an HTML block-level tag.

Indeed, most markdown implementations, including some of Gruber's
own perl implementations, do not impose these restrictions.

However, unlike Gruber's rule, this one requires that the open
tag be on a line by itself.  It also differs from most markdown
implementations in how it handles the case where there is no matching
closing tag (a case not mentioned in Gruber's rule).  In such a case,
the rule stated above includes the whole rest of the document in the
HTML block.