Let's say the user wants to use a larger font size on the LAST paragraph in the following example:

#section p { font-size: larger ! important}
.warning { color: red; border: thin solid red }
<div id="section">
<p>This is NOT a warning</p>
<p class="warning">This is a warning</p>

We're in a Wysiwyg environment, the user got the document from an unkown source, places the caret in the paragraph or selects the whole paragraph, click on some UI thingy meaning "I want a larger font" and he/she expects a larger font size for the paragraph, whatever has the editing tool to do for that... The user has no knowledge of the underlying technology - in our case CSS - and he/she's the perfect target for Wysiwyg editors. Let's suppose the editing tool manipulates stylesheets and hates HTML attributes and inline styles, and that's also our goal.

First thought and first problem, we cannot apply font-size: larger to the paragraph since it's already applied... But, wait, how do you know it's already applied? Just looking at the computed value of font-size, you can't since that will reply only an absolute value like 18px! So to determine if and how you can tweak a given property on a given element, you first need to browse all the CSS rules applying to your element. In Gecko, you can use the inIDOMUtils DOM Inspector interface and namely its getCSSStyleRules() method. Then for each rule applying to the element, you need to check if the property is applied to the element through that rule and if it is, compute the specificity of the selector and preserve the rule, the specificity of the rule and the priority (think !important) of the declaration in an array. When that's done for all rules, sort the array according to the CSS Cascade rules... Yep, that's right, you read it well, you need to recascade yourself the whole thing because the CSS Object Model has no getCascadedValue() method for the time being. Oh and of course, nothing either to compute the specificity of a given selector...

Once you have the cascaded the whole thing, you have the rule applying the current font-size to the element, its specificity and the priority of the declaration. Of course, there is a trivial case where no rule applies to the element for that property and your final data set is empty. 

Then the editing tool has no choice, it must create a rule with a specificity greater than or equal to the specificity of the rule we found and caring about the priority of the declaration...

  • If the data set above is empty or if the rule's specificity is equal to or less than the specificity of a class selector, AND IF the user says through some UI dialog we can safely apply the change to all <p class="warning">, then we can create a .warning { font-size: ...px } or .warning { font-size: ...px !important } rule at the end of our stylesheet where ... is a font-size value greater than the computed value of font-size on the element. Hey, yeah, you have to compute that value yourself again :-( We won't deal with the p.warning possibility because it's just impossible to show an average user a dialog asking for permission to change the styles applied to all paragraph of class warning and not other elements...
  • Else if the rule's specificity is equal to or less than the specificity of an #ID selector, we can safely insert a #myNewId { font-size: ...px }  or #myNewId { font-size: ...px ! important } at the end of our stylesheet but please note the editing tool has to query the user for an ID or select a random - and unreadable - one by itself. Of course, the font-size computation issue detailed just above still applies here.
  • Else if the rule's specificity is equal to or less than the specificity of an #ID selector and a type element selector, we can safely insert a p#myNewId { font-size: ...px } or p#myNewId { font-size: ...px ! important } at the end of our stylesheet. And the font-size computation and the ID UI query issues still apply here.
  • Else we have the most complex case: we need to prompt the user for an ID or find a random one and derive a rule from the selector of the rule we found above adding an ID simple selector to the last chain of simple selectors in the selector. In other terms in our case here, create #section p#myNewId { font-size: larger ! important} from #section p { font-size: larger ! important}.

Woof. Of course, the CSS Object Model's minimalistic interfaces don't help a lot here and the editing tool's author will end up writing a lot of code that is already implemented in the rendering engine, but not exposed. Welcome to my world :-)