GitHub Flavored Markdown (GFM), GitHub's proprietary version of the Markdown standard, has a couple of unsatisfactory features.
One is that there is no testing tool (such as the Markdown Dingus readily available, so the first time you write markdown you have to commit the file before you can see the result. That's bad design, since one the file is committed you can edit it on line and see what it looks like without committing again. So preview functionality exists; it just isn't available on the initial creation of a markdown file. And so the first time, you're stuck with whatever you produce. As a GitHub Flavored WorkAround I sometimes insert my new text into an existing markdown file and edit it until it looks the way I want it too, then copy it to the new file, canceling the changes to the existing file.
Another issue is that although you can turn soft-wrap on or off while editing; at present there doesn't seem to be a way to assign soft-wrap to one's default settings. I've written to GitHub tech support to suggest this, but the people who responded didn't seem to know what I was talking about and didn't follow up. I let it drop.
For a long time GFM didn't support HTML tables directly. You had to use a special GFM syntax for markdown tables, which has undocumented restrictions as to what you can do with it. You apparently cannot have merged cells or more than one hyphen-delimited line of header cells. Why is that important? Because it is very useful to be able to show a single merged header-cell representing two subordinate columns, each of which has its own subheader cell. For instance:
<table border="2" cellpadding="5" cellspacing="0"> <tr> <th rowspan="2">(empty)</th> <th rowspan="2">merged rows</th> <th colspan="2">merged columns</th> </tr> <tr> <th>unmerged</th> <th>unmerged</th> </tr> <tr> <th rowspan="2">merged rows</th> <td>row 1</td> <td>content 1</td> <td>content 2</td> </tr> <tr> <td>row 2</td> <td>content 1</td> <td>content 2</td> </tr> </table>
In HTML it produces:
|(empty)||merged rows||merged columns|
|merged rows||row 1||content 1||content 2|
|row 2||content 1||content 2|
But as I say, the good news is that HTML tables are now supported in GFM.
Since posting this, I've been told about Gollum, a Git-based wiki system that apparently supports GFM.