我编写了一个从 jQuery $(document).ready() 调用的 javascript 函数。这将解析父 div 的所有子元素,并且只更新最右边的子元素。
html
...
<div class="stretch">
<div style="padding-left: 5px; padding-right: 5px; display: inline-block;">Some text
</div>
<div class="underline" style="display: inline-block;">Some other text
</div>
</div>
....
javascript
$(document).ready(function(){
    stretchDivs();
});
function stretchDivs() {
    // loop thru each <div> that has class='stretch'
    $("div.stretch").each(function(){
        // get the inner width of this <div> that has class='stretch'
        var totalW = parseInt($(this).css("width"));
        // loop thru each child node
        $(this).children().each(function(){
            // subtract the margins, borders and padding
            totalW -= (parseInt($(this).css("margin-left")) 
                     + parseInt($(this).css("border-left-width")) 
                     + parseInt($(this).css("padding-left"))
                     + parseInt($(this).css("margin-right")) 
                     + parseInt($(this).css("border-right-width")) 
                     + parseInt($(this).css("padding-right")));
            // if this is the last child, we can set its width
            if ($(this).is(":last-child")) {
                $(this).css("width","" + (totalW - 1 /* fudge factor */) + "px");
            } else {
                // this is not the last child, so subtract its width too
                totalW -= parseInt($(this).css("width"));
            }
        });
    });
}