BookStack/resources/js/editor/schema-marks.js

import {schema as basicSchema} from "prosemirror-schema-basic";

const baseMarks = basicSchema.spec.marks;

const underline = {
    parseDOM: [{tag: "u"}, {style: "text-decoration=underline"}],
    toDOM() {
        return ["span", {style: "text-decoration: underline;"}, 0];
    }
};

const strike = {
    parseDOM: [{tag: "s"}, {tag: "strike"}, {style: "text-decoration=line-through"}],
    toDOM() {
        return ["span", {style: "text-decoration: line-through;"}, 0];
    }
};

const superscript = {
    parseDOM: [{tag: "sup"}],
    toDOM() {
        return ["sup", 0];
    }
};

const subscript = {
    parseDOM: [{tag: "sub"}],
    toDOM() {
        return ["sub", 0];
    }
};

const marks = baseMarks.append({
    underline,
    strike,
    superscript,
    subscript,
});

export default marks;