Hide ruby text (eg. furigana) on a web page. Mouseover to reveal.

hide-ruby-text.js

document.addEventListener("mouseover", (evt) =>
{
    const ruby = evt.target;

    if (ruby.tagName !== "RUBY")
        return;

    for (const rt of [...ruby.querySelectorAll("rt")])
        rt.style.opacity = "";
});

document.addEventListener("mouseout", (evt) =>
{
    const ruby = evt.target;

    if (ruby.tagName !== "RUBY")
        return;

    for (const rt of [...ruby.querySelectorAll("rt")])
        rt.style.opacity = "0";
});

const allRt = document.querySelectorAll("rt");
for (const rt of allRt)
{
    rt.style.opacity = "0";
}

Can be used as a bookmarklet (tested in Chrome and Safari). Just prepend the script text with javascript:.