This doesn't work well with scripts (and style elements) injected by the Hypothesis bookmarklet or the Wayback Machine's toolbar. On that note, it's pretty poor hygiene on their part to (a) inject this stuff in the body to begin with, and (b) not include at the very least a class attribute clearly defining the origin/role of the injected content. As I described elsewhere:
set the class on the injected element to an abbreviated address like <style class="example.org/sidebar/2.4/injected-content/">. And then drop a page there explaining the purpose and requirements (read: assumptions) of your injected element. This is virtually guaranteed not to conflict with any other class use (e.g. CSS rules in applied style sheets), and it makes it easier for other add-ons (or the page author or end user) to avoid conflicts with you.