* Move as much as possible to the CSS in the template.
* Ensure that all the HTML-based templates (including epub)
contain the CSS for columns.
* Columns default to 50% width unless they are given a width
attribute.
Closes#4028.
* Remove "width" attribute which is not allowed on div.
* Remove space between `<div class="column">` elements,
since this prevents columns whose widths sum to 100%
(the space takes up space).
Closes#4028.
The structure expected is:
<div class="columns">
<div class="column" width="40%">
contents...
</div>
<div class="column" width="60%">
contents...
</div>
</div>
Support has been added for beamer and all HTML slide formats.
Closes#1710.
Note: later we could add a more elegant way to create
this structure in Markdown than to use raw HTML div elements.
This would come for free with a "native div syntax" (#168).
Or we could devise something specific to slides
