FIX: server-side HtmlToMarkdown improvements (PR #9586)

TLDR; this commit vastly improves how whitespaces are handled when converting from HTML to Markdown. It also adds support for converting HTML <tables> to markdown tables.


The previous remove_whitespaces! method was traversing the whole HTML tree and used a heuristic to remove leading and trailing whitespaces whenever it was appropriate (ie. mostly before and after HTML block elements)

It was a good idea, but it was very limited and leaded to bad conversion when the html had leading whitespaces on several lines for example. One such example can be found here.

For various reasons, most of the whitespaces in a HTML file is ignored when the page is being displayed in a browser. The rules that the browsers follow are the CSS’ White Space Processing Rules. They can be quite complicated when you take into account RTL languages and other various tidbits but they boils down to the following:

  • Collapse whitespaces down to one space (0x20) inside an inline context (ie. nodes/tags that are being displaying on the same line)
  • Remove any leading/trailing whitespaces inside an inline context

One quick & dirty way of getting this 90% solved would be to do HTML.gsub!(/[[:space:]]+/, " "). We would also need to hoist <pre> elements in order to not mess with their whitespaces. Unfortunately, this solution let some whitespaces creep around HTML tags which leads to more .strip! calls than I can bear.

I decided to “emulate” the browser’s handling of whitespaces and came up with a solution in 4 parts

1. remove_not_allowed!

The HtmlToMarkdown library is recursively “visiting” all the nodes in the HTML in order to convert them to Markdown. All the nodes that aren’t handled by the library (eg. <script>, <style> or any non-textual HTML tags) are “swallowed”. In order to reduce the number of nodes visited, the method remove_not_allowed! will automatically delete all the nodes that have no “visitor” (eg. a visit_<tag> method) defined.

2. remove_hidden!

Similar purpose as the previous method (eg. reducing number of nodes visited), there’s no point trying to convert something that is hidden. The remove_hidden! method removes any nodes that was hidden using the “hidden” HTML attribute, some CSS or with a width or height equal to 0.

3. hoist_line_breaks!

The hoist_line_breaks! method is there to handle <br> tags. I know those tiny <br> don’t do much but they can be quite annoying. The <br> tags are inline elements but they visually work like a block element (ie. they create a new line). If you have the following HTML “<i>Foo<br>Bar</i>”, it ends up visually similar to “<i>Foo</i><br><i>Bar</i>”. The latter being much more easy to process than the former, so that’s what this method is doing. The hoist_line_breaks will hoist <br> tags out of inline tags until their parent is a block element.

4. remove_whitespaces!

The remove_whitespaces! is where all the whitespace removal is happening. It’s broken down into 4 methods as well

  • remove_whitespaces!
  • is_inline?
  • collapse_spaces!
  • remove_trailing_space!

The remove_whitespace! method is recursively walking the HTML tree (skipping <pre> tags). If a node has any children, they will be chunked into groups of inline elements vs block elements. For each chunks of inline elements, it will call the collapse_space! and remove_trailing_space! methods. For each chunks of block elements, it will call remote_whitespace! to keep walking the HTML tree recursively.

The is_inline? method determines whether a node is part of a inline context. A node is inline iif it’s a text node or it’s an inline tag, but not <br>, and all its children are also inline.

The collapse_spaces! method will collapse any kind of (white) space into a single space (" ") character, even across tags. For example, if we have “ Foo \n<i> Bar </i>\t42”, it will return “Foo <i>Bar </i>42”.

Finally, the remove_trailing_space! method is there to remove any trailing space that might creep in at the end of the inline chunk.

This solution is not 100% bullet-proof. It does not support RTL languages at all and has some caveats that I felt were not worth the work to get properly fixed.


FIX: switched Nokogiri to Nokogumbo for better HTML5 parsing FIX: better detection of hidden elements when converting HTML to Markdown FIX: take into account the allowed_href_schemes site setting when converting HTML <a> to Markdown FIX: added support for ‘mailto:’ scheme when converting <a> from HTML to Markdown FIX: added support for <img> dimensions when converting from HTML to Markdown FIX: added support for <dl>, <dd> and <dt> when converting from HTML to Markdown FIX: added support for multilines emphases, strongs and strikes when converting from HTML to Markdown FIX: added support for <acronym> when converting from HTML to Markdown DEV: remove unused ‘sanitize’ gem

Wow, did you just read all that?! Congratz, here’s a cookie: :cookie:.

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This was a huge commit but the approach seems good and I couldn’t spot anything out of the ordinary.

The tests are very nice!