diff --git a/data/sample.lua b/data/sample.lua
index ea20add2e..c0adae230 100644
--- a/data/sample.lua
+++ b/data/sample.lua
@@ -14,7 +14,7 @@
 -- syntax errors.
 
 local pipe = pandoc.pipe
-local stringify = (require "pandoc.utils").stringify
+local stringify = (require 'pandoc.utils').stringify
 
 -- The global variable PANDOC_DOCUMENT contains the full AST of
 -- the document which is going to be written. It can be used to
@@ -25,19 +25,19 @@ local meta = PANDOC_DOCUMENT.meta
 -- `image_format` meta value.
 local image_format = meta.image_format
   and stringify(meta.image_format)
-  or "png"
+  or 'png'
 local image_mime_type = ({
-    jpeg = "image/jpeg",
-    jpg = "image/jpeg",
-    gif = "image/gif",
-    png = "image/png",
-    svg = "image/svg+xml",
+    jpeg = 'image/jpeg',
+    jpg = 'image/jpeg',
+    gif = 'image/gif',
+    png = 'image/png',
+    svg = 'image/svg+xml',
   })[image_format]
-  or error("unsupported image format `" .. image_format .. "`")
+  or error('unsupported image format `' .. image_format .. '`')
 
 -- Character escaping
 local function escape(s, in_attribute)
-  return s:gsub("[<>&\"']",
+  return s:gsub('[<>&"\']',
     function(x)
       if x == '<' then
         return '&lt;'
@@ -60,7 +60,7 @@ end
 local function attributes(attr)
   local attr_table = {}
   for x,y in pairs(attr) do
-    if y and y ~= "" then
+    if y and y ~= '' then
       table.insert(attr_table, ' ' .. x .. '="' .. escape(y,true) .. '"')
     end
   end
@@ -72,7 +72,7 @@ local notes = {}
 
 -- Blocksep is used to separate block elements.
 function Blocksep()
-  return "\n\n"
+  return '\n\n'
 end
 
 -- This function is called once for the whole document. Parameters:
@@ -106,31 +106,31 @@ function Str(s)
 end
 
 function Space()
-  return " "
+  return ' '
 end
 
 function SoftBreak()
-  return "\n"
+  return '\n'
 end
 
 function LineBreak()
-  return "<br/>"
+  return '<br/>'
 end
 
 function Emph(s)
-  return "<em>" .. s .. "</em>"
+  return '<em>' .. s .. '</em>'
 end
 
 function Strong(s)
-  return "<strong>" .. s .. "</strong>"
+  return '<strong>' .. s .. '</strong>'
 end
 
 function Subscript(s)
-  return "<sub>" .. s .. "</sub>"
+  return '<sub>' .. s .. '</sub>'
 end
 
 function Superscript(s)
-  return "<sup>" .. s .. "</sup>"
+  return '<sup>' .. s .. '</sup>'
 end
 
 function SmallCaps(s)
@@ -142,33 +142,33 @@ function Strikeout(s)
 end
 
 function Link(s, tgt, tit, attr)
-  return "<a href='" .. escape(tgt,true) .. "' title='" ..
-         escape(tit,true) .. "'" .. attributes(attr) .. ">" .. s .. "</a>"
+  return '<a href="' .. escape(tgt,true) .. '" title="' ..
+         escape(tit,true) .. '"' .. attributes(attr) .. '>' .. s .. '</a>'
 end
 
 function Image(s, src, tit, attr)
-  return "<img src='" .. escape(src,true) .. "' title='" ..
-         escape(tit,true) .. "'/>"
+  return '<img src="' .. escape(src,true) .. '" title="' ..
+         escape(tit,true) .. '"/>'
 end
 
 function Code(s, attr)
-  return "<code" .. attributes(attr) .. ">" .. escape(s) .. "</code>"
+  return '<code' .. attributes(attr) .. '>' .. escape(s) .. '</code>'
 end
 
 function InlineMath(s)
-  return "\\(" .. escape(s) .. "\\)"
+  return '\\(' .. escape(s) .. '\\)'
 end
 
 function DisplayMath(s)
-  return "\\[" .. escape(s) .. "\\]"
+  return '\\[' .. escape(s) .. '\\]'
 end
 
 function SingleQuoted(s)
-  return "&lsquo;" .. s .. "&rsquo;"
+  return '&lsquo;' .. s .. '&rsquo;'
 end
 
 function DoubleQuoted(s)
-  return "&ldquo;" .. s .. "&rdquo;"
+  return '&ldquo;' .. s .. '&rdquo;'
 end
 
 function Note(s)
@@ -184,11 +184,11 @@ function Note(s)
 end
 
 function Span(s, attr)
-  return "<span" .. attributes(attr) .. ">" .. s .. "</span>"
+  return '<span' .. attributes(attr) .. '>' .. s .. '</span>'
 end
 
 function RawInline(format, str)
-  if format == "html" then
+  if format == 'html' then
     return str
   else
     return ''
@@ -200,8 +200,8 @@ function Cite(s, cs)
   for _,cit in ipairs(cs) do
     table.insert(ids, cit.citationId)
   end
-  return "<span class=\"cite\" data-citation-ids=\"" .. table.concat(ids, ",") ..
-    "\">" .. s .. "</span>"
+  return '<span class="cite" data-citation-ids="' .. table.concat(ids, ',') ..
+    '">' .. s .. '</span>'
 end
 
 function Plain(s)
@@ -209,16 +209,16 @@ function Plain(s)
 end
 
 function Para(s)
-  return "<p>" .. s .. "</p>"
+  return '<p>' .. s .. '</p>'
 end
 
 -- lev is an integer, the header level.
 function Header(lev, s, attr)
-  return "<h" .. lev .. attributes(attr) ..  ">" .. s .. "</h" .. lev .. ">"
+  return '<h' .. lev .. attributes(attr) ..  '>' .. s .. '</h' .. lev .. '>'
 end
 
 function BlockQuote(s)
-  return "<blockquote>\n" .. s .. "\n</blockquote>"
+  return '<blockquote>\n' .. s .. '\n</blockquote>'
 end
 
 function HorizontalRule()
@@ -234,39 +234,39 @@ function CodeBlock(s, attr)
   -- If code block has class 'dot', pipe the contents through dot
   -- and base64, and include the base64-encoded png as a data: URL.
   if attr.class and string.match(' ' .. attr.class .. ' ',' dot ') then
-    local img = pipe("base64", {}, pipe("dot", {"-T" .. image_format}, s))
+    local img = pipe('base64', {}, pipe('dot', {'-T' .. image_format}, s))
     return '<img src="data:' .. image_mime_type .. ';base64,' .. img .. '"/>'
   -- otherwise treat as code (one could pipe through a highlighter)
   else
-    return "<pre><code" .. attributes(attr) .. ">" .. escape(s) ..
-           "</code></pre>"
+    return '<pre><code' .. attributes(attr) .. '>' .. escape(s) ..
+           '</code></pre>'
   end
 end
 
 function BulletList(items)
   local buffer = {}
   for _, item in pairs(items) do
-    table.insert(buffer, "<li>" .. item .. "</li>")
+    table.insert(buffer, '<li>' .. item .. '</li>')
   end
-  return "<ul>\n" .. table.concat(buffer, "\n") .. "\n</ul>"
+  return '<ul>\n' .. table.concat(buffer, '\n') .. '\n</ul>'
 end
 
 function OrderedList(items)
   local buffer = {}
   for _, item in pairs(items) do
-    table.insert(buffer, "<li>" .. item .. "</li>")
+    table.insert(buffer, '<li>' .. item .. '</li>')
   end
-  return "<ol>\n" .. table.concat(buffer, "\n") .. "\n</ol>"
+  return '<ol>\n' .. table.concat(buffer, '\n') .. '\n</ol>'
 end
 
 function DefinitionList(items)
   local buffer = {}
   for _,item in pairs(items) do
     local k, v = next(item)
-    table.insert(buffer, "<dt>" .. k .. "</dt>\n<dd>" ..
-                   table.concat(v, "</dd>\n<dd>") .. "</dd>")
+    table.insert(buffer, '<dt>' .. k .. '</dt>\n<dd>' ..
+                   table.concat(v, '</dd>\n<dd>') .. '</dd>')
   end
-  return "<dl>\n" .. table.concat(buffer, "\n") .. "\n</dl>"
+  return '<dl>\n' .. table.concat(buffer, '\n') .. '\n</dl>'
 end
 
 -- Convert pandoc alignment to something HTML can use.
@@ -303,13 +303,13 @@ function Table(caption, aligns, widths, headers, rows)
   local function add(s)
     table.insert(buffer, s)
   end
-  add("<table>")
-  if caption ~= "" then
-    add("<caption>" .. escape(caption) .. "</caption>")
+  add('<table>')
+  if caption ~= '' then
+    add('<caption>' .. escape(caption) .. '</caption>')
   end
   if widths and widths[1] ~= 0 then
     for _, w in pairs(widths) do
-      add('<col width="' .. string.format("%.0f%%", w * 100) .. '" />')
+      add('<col width="' .. string.format('%.0f%%', w * 100) .. '" />')
     end
   end
   local header_row = {}
@@ -317,7 +317,7 @@ function Table(caption, aligns, widths, headers, rows)
   for i, h in pairs(headers) do
     local align = html_align(aligns[i])
     table.insert(header_row,'<th align="' .. align .. '">' .. h .. '</th>')
-    empty_header = empty_header and h == ""
+    empty_header = empty_header and h == ''
   end
   if not empty_header then
     add('<tr class="header">')
@@ -326,9 +326,9 @@ function Table(caption, aligns, widths, headers, rows)
     end
     add('</tr>')
   end
-  local class = "even"
+  local class = 'even'
   for _, row in pairs(rows) do
-    class = (class == "even" and "odd") or "even"
+    class = (class == 'even' and 'odd') or 'even'
     add('<tr class="' .. class .. '">')
     for i,c in pairs(row) do
       add('<td align="' .. html_align(aligns[i]) .. '">' .. c .. '</td>')
@@ -340,7 +340,7 @@ function Table(caption, aligns, widths, headers, rows)
 end
 
 function RawBlock(format, str)
-  if format == "html" then
+  if format == 'html' then
     return str
   else
     return ''
@@ -348,7 +348,7 @@ function RawBlock(format, str)
 end
 
 function Div(s, attr)
-  return "<div" .. attributes(attr) .. ">\n" .. s .. "</div>"
+  return '<div' .. attributes(attr) .. '>\n' .. s .. '</div>'
 end
 
 -- The following code will produce runtime warnings when you haven't defined
@@ -358,6 +358,6 @@ local meta = {}
 meta.__index =
   function(_, key)
     io.stderr:write(string.format("WARNING: Undefined function '%s'\n",key))
-    return function() return "" end
+    return function() return '' end
   end
 setmetatable(_G, meta)
diff --git a/test/writer.custom b/test/writer.custom
index ce490f426..eb53363fa 100644
--- a/test/writer.custom
+++ b/test/writer.custom
@@ -5,7 +5,7 @@ John Gruber’s markdown test suite.</p>
 
 <h1 id="headers">Headers</h1>
 
-<h2 id="level-2-with-an-embedded-link">Level 2 with an <a href='/url' title=''>embedded link</a></h2>
+<h2 id="level-2-with-an-embedded-link">Level 2 with an <a href="/url" title="">embedded link</a></h2>
 
 <h3 id="level-3-with-emphasis">Level 3 with <em>emphasis</em></h3>
 
@@ -525,7 +525,7 @@ Blah
 
 <p>This is <strong>strong</strong>, and so <strong>is this</strong>.</p>
 
-<p>An <em><a href='/url' title=''>emphasized link</a></em>.</p>
+<p>An <em><a href="/url" title="">emphasized link</a></em>.</p>
 
 <p><strong><em>This is strong and em.</em></strong></p>
 
@@ -560,7 +560,7 @@ So is &lsquo;pine.&rsquo;</p>
 <p>&lsquo;He said, &ldquo;I want to go.&rdquo;&rsquo; Were you alive in the
 70’s?</p>
 
-<p>Here is some quoted &lsquo;<code>code</code>&rsquo; and a &ldquo;<a href='http://example.com/?foo=1&amp;bar=2' title=''>quoted link</a>&rdquo;.</p>
+<p>Here is some quoted &lsquo;<code>code</code>&rsquo; and a &ldquo;<a href="http://example.com/?foo=1&amp;bar=2" title="">quoted link</a>&rdquo;.</p>
 
 <p>Some dashes: one—two — three—four — five.</p>
 
@@ -660,70 +660,70 @@ So is &lsquo;pine.&rsquo;</p>
 
 <h2 id="explicit">Explicit</h2>
 
-<p>Just a <a href='/url/' title=''>URL</a>.</p>
+<p>Just a <a href="/url/" title="">URL</a>.</p>
 
-<p><a href='/url/' title='title'>URL and title</a>.</p>
+<p><a href="/url/" title="title">URL and title</a>.</p>
 
-<p><a href='/url/' title='title preceded by two spaces'>URL and title</a>.</p>
+<p><a href="/url/" title="title preceded by two spaces">URL and title</a>.</p>
 
-<p><a href='/url/' title='title preceded by a tab'>URL and title</a>.</p>
+<p><a href="/url/" title="title preceded by a tab">URL and title</a>.</p>
 
-<p><a href='/url/' title='title with &quot;quotes&quot; in it'>URL and title</a></p>
+<p><a href="/url/" title="title with &quot;quotes&quot; in it">URL and title</a></p>
 
-<p><a href='/url/' title='title with single quotes'>URL and title</a></p>
+<p><a href="/url/" title="title with single quotes">URL and title</a></p>
 
-<p><a href='/url/with_underscore' title=''>with_underscore</a></p>
+<p><a href="/url/with_underscore" title="">with_underscore</a></p>
 
-<p><a href='mailto:nobody@nowhere.net' title=''>Email link</a></p>
+<p><a href="mailto:nobody@nowhere.net" title="">Email link</a></p>
 
-<p><a href='' title=''>Empty</a>.</p>
+<p><a href="" title="">Empty</a>.</p>
 
 <h2 id="reference">Reference</h2>
 
-<p>Foo <a href='/url/' title=''>bar</a>.</p>
+<p>Foo <a href="/url/" title="">bar</a>.</p>
 
-<p>With <a href='/url/' title=''>embedded [brackets]</a>.</p>
+<p>With <a href="/url/" title="">embedded [brackets]</a>.</p>
 
-<p><a href='/url/' title=''>b</a> by itself should be a link.</p>
+<p><a href="/url/" title="">b</a> by itself should be a link.</p>
 
-<p>Indented <a href='/url' title=''>once</a>.</p>
+<p>Indented <a href="/url" title="">once</a>.</p>
 
-<p>Indented <a href='/url' title=''>twice</a>.</p>
+<p>Indented <a href="/url" title="">twice</a>.</p>
 
-<p>Indented <a href='/url' title=''>thrice</a>.</p>
+<p>Indented <a href="/url" title="">thrice</a>.</p>
 
 <p>This should [not][] be a link.</p>
 
 <pre><code>[not]: /url</code></pre>
 
-<p>Foo <a href='/url/' title='Title with &quot;quotes&quot; inside'>bar</a>.</p>
+<p>Foo <a href="/url/" title="Title with &quot;quotes&quot; inside">bar</a>.</p>
 
-<p>Foo <a href='/url/' title='Title with &quot;quote&quot; inside'>biz</a>.</p>
+<p>Foo <a href="/url/" title="Title with &quot;quote&quot; inside">biz</a>.</p>
 
 <h2 id="with-ampersands">With ampersands</h2>
 
-<p>Here’s a <a href='http://example.com/?foo=1&amp;bar=2' title=''>link with an ampersand in the URL</a>.</p>
+<p>Here’s a <a href="http://example.com/?foo=1&amp;bar=2" title="">link with an ampersand in the URL</a>.</p>
 
-<p>Here’s a link with an amersand in the link text: <a href='http://att.com/' title='AT&amp;T'>AT&amp;T</a>.</p>
+<p>Here’s a link with an amersand in the link text: <a href="http://att.com/" title="AT&amp;T">AT&amp;T</a>.</p>
 
-<p>Here’s an <a href='/script?foo=1&amp;bar=2' title=''>inline link</a>.</p>
+<p>Here’s an <a href="/script?foo=1&amp;bar=2" title="">inline link</a>.</p>
 
-<p>Here’s an <a href='/script?foo=1&amp;bar=2' title=''>inline link in pointy braces</a>.</p>
+<p>Here’s an <a href="/script?foo=1&amp;bar=2" title="">inline link in pointy braces</a>.</p>
 
 <h2 id="autolinks">Autolinks</h2>
 
-<p>With an ampersand: <a href='http://example.com/?foo=1&amp;bar=2' title='' class="uri">http://example.com/?foo=1&amp;bar=2</a></p>
+<p>With an ampersand: <a href="http://example.com/?foo=1&amp;bar=2" title="" class="uri">http://example.com/?foo=1&amp;bar=2</a></p>
 
 <ul>
 <li>In a list?</li>
-<li><a href='http://example.com/' title='' class="uri">http://example.com/</a></li>
+<li><a href="http://example.com/" title="" class="uri">http://example.com/</a></li>
 <li>It should.</li>
 </ul>
 
-<p>An e-mail address: <a href='mailto:nobody@nowhere.net' title='' class="email">nobody@nowhere.net</a></p>
+<p>An e-mail address: <a href="mailto:nobody@nowhere.net" title="" class="email">nobody@nowhere.net</a></p>
 
 <blockquote>
-<p>Blockquoted: <a href='http://example.com/' title='' class="uri">http://example.com/</a></p>
+<p>Blockquoted: <a href="http://example.com/" title="" class="uri">http://example.com/</a></p>
 </blockquote>
 
 <p>Auto-links should not occur here: <code>&lt;http://example.com/&gt;</code></p>
@@ -740,7 +740,7 @@ So is &lsquo;pine.&rsquo;</p>
 <img src="lalune.jpg" id="" alt="lalune"/><figcaption>lalune</figcaption>
 </figure>
 
-<p>Here is a movie <img src='movie.jpg' title=''/> icon.</p>
+<p>Here is a movie <img src="movie.jpg" title=""/> icon.</p>
 
 <hr/>
 
@@ -774,7 +774,7 @@ footnote (as with list items).</p>
 lazy and just indent the first line of each block. <a href="#fnref2">&#8617;</a></p></li>
 <li id="fn3"><p>This
 is <em>easier</em> to type. Inline notes may contain
-<a href='http://google.com' title=''>links</a> and <code>]</code> verbatim characters,
+<a href="http://google.com" title="">links</a> and <code>]</code> verbatim characters,
 as well as [bracketed text]. <a href="#fnref3">&#8617;</a></p></li>
 <li id="fn4"><p>In quote. <a href="#fnref4">&#8617;</a></p></li>
 <li id="fn5"><p>In list. <a href="#fnref5">&#8617;</a></p></li>