给输入框添加keydown事件，进行判断，如果按下的是tab键，则阻止默认的行为。

測試通過，行爲於 SF 編輯器幾乎一致（"\t" 改成 " " 就完全一致了）。

順便吐槽 SF 編輯器選中多行文本沒有像 Sublime Text 那樣自動每行縮進一格。

var el = document.querySelector('textarea');

el.addEventListener("keydown", function(e) {
    var keyCode = e.keyCode || e.which;

    if (keyCode === 9) {
        var start = el.selectionStart,
            end = el.selectionEnd;

        el.value = el.value.substring(0, start)
                + "\t"
                + el.value.substring(end);

        el.selectionStart = el.selectionEnd = start + 1;

        e.preventDefault();
    }
})

參見：http://stackoverflow.com/questions/6140632/how-to-handle-tab-in-textarea

想要 Sublime Text 一樣選中多行文本按 Tab 時每行縮進一 Tab 的話可以用這段：

var el = document.querySelector('textarea');

el.addEventListener("keydown", function(e) {
    var keyCode = e.keyCode || e.which;

    if (keyCode === 9) {
        var start = el.selectionStart,
            end = el.selectionEnd, 
            value = el.value;

        var lineStart = value.lastIndexOf('\n', start),
            lineEnd = value.indexOf('\n', end),
            offset = 0;

        if (lineStart === -1) lineStart = 0;
        if (lineEnd === -1) lineEnd = value.length;

        if (lineStart === lineEnd);
        else if (lineStart !== 0) lineStart += 1;

        lines = value.substring(lineStart, lineEnd).split('\n');
        console.log([lineStart, lineEnd], lines);

        if (lines.length > 1) {
            offset = lines.length;
            lines = '\t' + lines.join('\n\t');

            el.value = value.substring(0, lineStart) + lines + value.substring(lineEnd);

            el.selectionStart = start + 1;
            el.selectionEnd = end + offset;
        } else {
            offset = 1;
            lines = lines[0];

            el.value = value.substring(0, start) + '\t' + value.substring(end);

            el.selectionStart = el.selectionEnd = start + offset;
        }

        e.preventDefault();
    }
})

keydown事件,判断键值是否是9,如果是,则实现缩进,可以使用4个空格或者水平制表符:\t