Do highlighting for at least the navigation node

NVDA has the ability to highlight the current focus (Visual preference: highlight system focus). Maybe this can be done while navigating. It requires adding `id`s to the web page, or writing some code that maps the internal ids used for reporting the current node back to the original tree. Then JS can get the bounding box and highlight it. CSS could be used if modifying the document is an option.

首先追蹤導覽如何回報目前的節點，以及 MathCAT 輸出如何整合到網頁中。比較提議的 internal-ID 對映與將 ID 加入文件的做法，然後確認 JavaScript 能夠定位該節點，且其 bounding box 會在導覽期間反白顯示。

由索引模型根據 Issue 內容生成。