find(query, context = this.rootEl ?? this._document) {
    const result = this.maybeFind(query, context);
    if (result === null) {
      throw new Error(`query ${query} not found`);
    }

    return result;
  }
Typed and guaranteed context.querySelector. Always returns an element or throws if
nothing matches query.

@template {string} T
@param {T} query
@param {ParentNode} context
@return {ParseSelector<T>}