user13877: What I do:
$ ffmpeg -i a.mkv -c:s srt b.srt

The resulting file contains HTML tags, is there a way to avoid that?
On a side note, where can I find the list of... what do they call it... encoders? I simply tried srt for the name and it worked.

user29988: Instead of srt try to use text:
$ ffmpeg -i a.mkv -c:s text b.srt

(source)
For a list of all codecs use:
ffmpeg -encoders

You can check the documentation for more options.

13877: <p>What I do:</p>
<pre><code>$ ffmpeg -i a.mkv -c:s srt b.srt
</code></pre>
<p>The resulting file contains HTML tags, is there a way to avoid that?</p>
<p>On a side note, where can I find the list of... what do they call it... encoders? I simply tried <code>srt</code> for the name and it worked.</p>
 29988: <p>Instead of <em>srt</em> try to use <em>text</em>:</p>
<pre><code>$ ffmpeg -i a.mkv -c:s text b.srt
</code></pre>
<p><a href="https://ffmpeg.org/pipermail/ffmpeg-user/2015-December/029668.html" rel="nofollow noreferrer">(source)</a></p>
<p>For a list of all codecs use:</p>
<pre><code>ffmpeg -encoders
</code></pre>
<p>You can check the <a href="https://ffmpeg.org/ffmpeg.html" rel="nofollow noreferrer">documentation</a> for more options.</p>
