How to customize the appearance of an element within a shadow DOM

I am currently implementing a custom CSS style over an application without the ability to modify its code.

Initially, the DOM structure looked like this:

<div class="foo">
  <div class="bar"></div>
</div>

and I could easily make modifications using .foo .bar.

However, with the latest version release of the app, the child element is now within a shadow DOM:

<div class="foo">
  #shadow-root
    <div class="bar"></div>
</div>

As expected, my previous selector no longer functions. How can I target the .bar element only if it is nested inside a .foo element? I have come across information about ::part(), but unfortunately, I am unable to add part="bar" directly to the DOM element. Thank you for any assistance provided.

Answer №1

When using only CSS and not utilizing the part attribute, it becomes challenging to select child elements from an external source. This intentional limitation allows creators of Shadow DOM to maintain full control over which elements can be styled externally, as outlined in the motivation.

However, if JavaScript is permitted and the Shadow DOM is established with an 'open' mode, you have the ability to target child elements using shadowRoot.querySelector.

const host = document.querySelector("#host");
const shadow = host.attachShadow({ mode: "open" });
const div = document.createElement("div");
div.className = "bar";
div.textContent = "I'm in the shadow DOM";
shadow.appendChild(div);


document.querySelector("#host").shadowRoot.querySelector(".bar").style.color = "red";
<div id="host" class="foo"></div>

You can also dynamically include styles that target the bar element through JavaScript by following guidelines like this one, but this approach is only viable when the mode is set to open.

If not, you may need to investigate whether custom properties are being used in the app's styles that require customization or carefully design the styling of the Shadow DOM with inherited properties.

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