CustomStyles strange behaviour

I have been messing around with the CustomStyles style injector for an addon and am getting some strange behaviour.

I inject the below code during gui_hooks.editor_will_load_note:

(async () => {
    while (!require("anki/RichTextInput").instances?.length) {
        await new Promise(requestAnimationFrame);

    for (const { customStyles } of require("anki/RichTextInput").instances) {
        const { addStyleTag } = await customStyles;
        const { element } = await addStyleTag("chineseSupport");

If this code above runs before the code to apply the userBase style runs:

anki/ts/editor/rich-text-input/RichTextStyles.svelte at main · ankitects/anki (

then there are no issues. However, if this code executes after the code applying the userBase style, the styling from the <style id="userBase"> tag is wiped out.

Interestingly, if we change how the userBase style is applied and have it set the actual textContent of the <style> tag, then the styling is respected and the bug disappears. Something like this:

async function setStyling(property: string, value: unknown): Promise<void> {
    const rule = await userBaseRule;[property] = value;

    const baseStyle = await userBaseStyle;
    baseStyle.element.textContent = rule.cssText;

anki/ts/editor/rich-text-input/RichTextStyles.svelte at main · ankitects/anki (

Maybe svelte is getting confused with the dynamically changing styles, maybe there is a bug in the webengine itself?

I wanted to see if anyone else has experienced this issue or has any knowledge of it / mitigating it.

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