It has nothing to do with id selectors, here's an example without id selectors
Code:
p span.test {
color: red
}
<p>
<span class="test" style="color:green">test</span>
</p>
The point is that there is no way to map the specificity of an inline style to a selector based style robustly. Which means that the CSS cascade before and after the plugin runs is different.