How To Isolate a div from public CSS styles?

Old question, but things have changed a little bit since the accepted answer. There is now a CSSWG-recommended keyword called revert which would be better suited than initial and unset to solve this problem, as it resets properties to what they're defined to in the user agent stylesheet, rather than to their initial value (which has no regard for which element they're used on). So for instance, with revert, a div inside #mydiv will have its display set to block as we would expect and not inline (the initial value of display).

You'd have to do this:

#mydiv,
#mydiv::before,
#mydiv::after,
#mydiv *
#mydiv *::before,
#mydiv *::after {
  all: revert;
}

At the time of writing, revert is supported in Edge, Firefox, Chrome, and Safari, but not IE or Opera.

There is also something else to take into consideration regarding the accepted answer. If you want to style anything inside #mydiv, you need to do so with a selector that is at least as specific as the one you used to unset or revert everything, otherwise it will be overridden by that rule, even if it comes after it in the CSS.

So you'd need to do something like this (note the #mydiv which boost the specificity of the rules):

#mydiv p {
  margin-top: 20px;
}

#mydiv .bg-black {
  background-color: black;
}

// etc.

CSS Cascading and Inheritance Level 3 introduces the all shorthand property and the unset keyword, which, together, allow you to achieve this conveniently.

For example, if an author specifies all: initial on an element it will block all inheritance and reset all properties, as if no rules appeared in the author, user, or user-agent levels of the cascade.

This can be useful for the root element of a "widget" included in a page, which does not wish to inherit the styles of the outer page. Note, however, that any "default" style applied to that element (such as, e.g. display: block from the UA style sheet on block elements such as <div>) will also be blown away.

You’ll need to apply all: initial to your div and all: unset to its descendants:

#mydiv {
  all: initial; /* blocking inheritance for all properties */
}
#mydiv * {
  all: unset; /* allowing inheritance within #mydiv */
}

You may want to use a class on your div instead of an id, so that any rules you write to style its descendants won’t have to match or beat the high specificity used in this rule.

To be really safe, you may want to block styles on potential pseudo-element descendants too:

#mydiv::before,
#mydiv::after,
#mydiv *::before,
#mydiv *::after {
  all: unset;
}

Alternatively, for broader browser support, you can manually attempt to do what all does by setting all known CSS properties (don’t forget the prefixed versions):

#mydiv {
  /*
   * using initial for all properties
   * to totally block inheritance
   */
  align-content: initial;
  align-items: initial;
  align-self: initial;
  alignment-baseline: initial;
  animation: initial;
  backface-visibility: initial;
  background: initial;
  ...
}

#mydiv::before,
#mydiv::after,
#mydiv *,
#mydiv *::before,
#mydiv *::after {
  /*
   * using inherit for normally heritable properties,
   * and initial for the others, as unset does
   */
  align-content: initial;
  align-items: initial;
  align-self: initial;
  ...
  color: inherit;
  ...
}

You can encourage browser support for the all shorthand property and track its adoption with these issue links:

• ☑ Chrome 37+
• ☑ Firefox 27+
• ☑ Webkit (Safari 9.1+)
• ☐ Internet Explorer
• ☑ Edge 79+
• ☑ Opera 24+

Up-to-date browser support information for the all shorthand property is available here.