Inconsistent escape of image alt

[xiaq]

An obscure edge case, but still...

< is usually escaped in image alt:

![a <](b.png)

becomes

<p><img src="b.png" alt="a &lt;" /></p>

(https://spec.commonmark.org/dingus/?text=!%5Ba%20%3C%5D(b.png))

However, if the < is part of an HTML tag, it is not escaped in the output:

![a <a>](b.png)

becomes

<p><img src="b.png" alt="a <a>" /></p>

(https://spec.commonmark.org/dingus/?text=!%5Ba%20%3Ca%3E%5D(b.png))

cmark generates the expected output:

<p><img src="b.png" alt="a &lt;a&gt;" /></p>