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Protect element from global css

My js is embedded on a third party website and it creates an <iframe> which contains a simple comments panel , but apparently on this specific website there is a CSS stylesheet which styles every <iframe> tag in the dom with the !important flag , so i can't change those css rules and the website dev team won't change this behaviour, there is another way to overcome this? can i change the tagname and still be an iframe? anything?

You can use the all property with the initial value to default the styles for that element.
From the docs:
The all CSS shorthand property sets all of an element's properties
(other than unicode-bidi and direction) to their initial or inherited
values, or to the values specified in another stylesheet origin.
A code example:
#div-to-reset-styles {
all: initial;
* {
all: unset;
}
}
Just target your specific iframe and you should be fine.

CSS Specificity is your friend here. From this overview:
Specificity determines, which CSS rule is applied by the browsers.
Specificity is usually the reason why your CSS-rules don’t apply to some elements, although you think they should.
Every selector has its place in the specificity hierarchy.
If two selectors apply to the same element, the one with higher specificity wins.
There are four distinct categories which define the specificity level of a given selector: inline styles, IDs, classes, attributes, and elements.
...
The part in bold means that you can add a class to your element(s) in question and override the more generic iframe css definition like that.

Change css property on loading a page using JQuery or JavaScript

How can I setup to change a border-color( css property ) of my .gallery( css class) to white, instead of black, when page loads?
I want this property to be the final, no override by index css file or any other css file.

the simples way would be:
$(document).ready(function() {
$('.gallery').css('border-color', '#ffffff')
});
This sets the style attribute of your node, overriding any properties from your css (unless they are set to !important)

How to apply function to all elements having border-radius property

The idea is making some border-radius effect in IE 7/8, so I've decided to use jquery.corner.js library. To make it more generic I want to write some script which applies corner() function to all elements within a page having border-radius property.
For example, for this element
.someElement
{
border-radius:10px;
}
function must do the following
$(".someElement").corner("10px");
The problem is that I want to apply rounded corners to all elements, including dynamically added elements and elements which are inheriting border-radius property among some action(hover, click, etc.). Is this possible?

You need to declare a function that applies you css on every change.
To detect css style changes, see here:
Event detect when css property changed using Jquery
Then you need call that function on style change and on dom tree change (every time you append something into the page)....
I would advise you use a specific class to apply border radius css. This way you can select the rounded elements via jQuery class selectors.

You should have a generic css class that is used on all elements that have rounded borders and then use that class in your selector.
You will have to do this in a document ready handler. This will of course only apply rounded borders to elements that currently exists. If you want to cover elements loaded with ajax you can do the following:
$(document).ajaxSuccess(function(e, xhr, settings)
{
$(xhr.responseText).find(".class-that-applies-rounded-borders").corner("10px");
});

jQuery Plugin to limit styles to a specifc area

I'm searching for something that applies styles only to a specific area:
<div class="showcase limited-styles">
<style scoped type="text/css">/* some styles */</style>
<!-- the actual content - only affected by the style definitions above -->
</div>
I already found jQuery Scoped, but for some reason it doesn't work for me - all the other styles apply as well, so it's not any better as using the highest available specifity.
Thanks!

It seems like you could accomplish this by "parenting"
Throw a containing div around all of the elements you want to respond independently, and then put that div name in front of other DOM elements you are trying to style.
Here is a fiddle for reference http://jsfiddle.net/3mx4L07t/

Remove/change Internal styles sheet styles with JavaScript/jQuery

I want to be able to remove/change the internal style sheets values through JavasScript. It has to be through JS because I cannot edit the html since I am using an application that does not allow me to, but I can use JS. How can I change the value for the background or remove it completely? Or is there another way to accomplish this?
<body>
<head>
<style type="text/css">
#company-header{background:#000 !important;}
</style>
</head>
<div id="company-header"></div>
</body>

If your just going to change small bits of css, using jQuery's css() is your best option, but it does not always recognize !important, for that you would probably need cssText, like this:
$('#id').css('cssText', 'height: 200px !important');
This replaces all the css, so everything needs to be defined in the new css rules that are added.
If you are changing a lot of css, or just want to make it easier for the next time, you could remove all inline css and add an external stylesheet instead.
To remove all inline styles you would do something like this:
$(document).removeAttr('style');
or for div's only
$('div').removeAttr('style');
Depending on how many styles there are, this could take som time to process.
Then to add a new stylesheet do:
(function() {
var myCSS = document.createElement('link');
myCSS.rel = 'stylesheet';
myCSS.type = 'text/css';
myCSS.src = '/styles/mystylesheet.css';
var place = document.getElementsByTagName('script')[0];
place.parentNode.insertBefore(myCSS, place);
})();
Edit:
function removeCSS() {
var badCSS = document.getElementsByTagName('style')[0];
$(badCSS).remove();
});
This will remove all the markup in the internal stylesheet, but since the styles are already loaded it will make absolutely no difference.
Internal styles will always override external styles, but for one exeption, if the external style is !important. If both the external and internal styles are !important, the internal style will be used.
Loading an external stylesheet dynamicly with javascript will only work if everything you are trying to override is set to !important in the external stylesheet, and not set to !important in the internal stylesheet.
Changing the styles directly in the DOM with jQuery's css() and using the cssText option to override the !important set in the internal stylesheet may be your only viable option if there is absolutely no way to alter the html file and remove the internal stylesheet.

EDIT: OK, now that we understand that this question is really just about how to override the !important style declaration when setting styles via javascript, this is a duplicate of this posting: Overriding !important style.
The two possible answers there are to set a whole style string on the object or to create a new stylesheet that refers to this object.
Previous answer before question was edited:
You can just set some style directly on the object if you want like this. This will override anything that comes from a style sheet so you don't have to mess with the style sheet.
$("#company-header").css("background-color", "#FFFFFF");
or
$("#company-header").css("background", "none");
A more extensible way of modifying the look of an object or groups of objects is to based the style settings on class names and then add/remove class names using jQuery. This has the effect of switching which style sheet rules apply to a given object without having to directly manipulate the style sheets themselves:
$("#company-header").removeClass("oldClass").addClass("newClass");

$(document).ready(function(){
$('style').remove();
});
This code will remove the all internal css from the site. I used this but style tag will be visible in the page source.