If you happen to be using sass in a build system, one way to do this that will work in all the major browsers is to wrap all your style imports with a :not() selector like so...
:not(.disable-all-styles) {
@import 'my-sass-file';
@import 'my-other-sass-file';
}
Then you can use the disable class on a container and the sub-content won't have any of your styles.
<div class="disable-all-styles">
<p>Nothing in this div is affected by my sass styles.</p>
</div>
Of course all your styles will now be prepended with the :not() selector, so it's a little fugly, but works well.