Priority incorrect on CSS inline class

If you don't want to have them overwritten you first need to obtain the existing properties from the HTML source.

You then can either check if those are already set before setting the new to prevent overwriting, or you just overwrite them later on to the default ones.

As existing properties are already extracted before, it might work as:

foreach($rule['properties'] as $key => $value)
    if(!isset($properties[$key])) $properties[$key] = $value;

(this will overwrite NULL properties, in case it's a problem, use array_key_exists instead of isset).

You should add a flag to the class/function to change the behavior, so the current behavior does not break but you make your needs optional.

Edit: As multiple rules can apply, the next rule will not overwrite something of the previous rule (which it should but is prevented by the edit above), so this needs more control to preserve the original properties:

$originalProperties = $properties;
foreach($rule['properties'] as $key => $value) $properties[$key] = $value;
$properties = $originalProperties + $properties;

Edit 2: The second suggestion won't work as well. Basically there is need to get the very original values because that function changes the style attribute while it iterates over rules and elements. The same element can become modified twice, so the $originalProperties are not really the original properties but can be modified ones.

Maybe this works with spl_object_hash to create a consistent id/key for each element, but I'm not sure. Maybe DomElement offers something comparable internally, but I have not found such (which should fit better). With a specific key/id per each concrete element, only on first appearance, the very original properties can be preserved:

$elementId = spl_object_hash($element);
if(!isset($originalProperties[$elementId]))
{
    $originalProperties[$elementId] = $properties;
    $originalElements[$elementId] = $element; // Needed to make spl_object_hash work
}
foreach($rule['properties'] as $key => $value) $properties[$key] = $value;
$properties = $originalProperties[$elementId] + $properties;

You can use !important after the CSS definition to make sure it takes priority.

E.g.

.yourstyle
{
    color: #000 !important;
}