当屏幕上只有一个计数器时工作正常，但我如何修改此代码以使其同时为屏幕上的多个计数器工作......
1.将HTML元素 Package 到div中以创建多个块。如果您想单独针对这些块，则给予此 Package 器id。或者，如果您想针对所有块，则只需给予相同的class。
1.将整个代码 Package 到一个函数中。这将封装逻辑。
1.使用元素id或元素本身调用该函数。
1.在函数中获取元素中span s的引用。这可以通过class或nth-child选择器完成。

示例：

• 下面的示例是您的原始代码，只是按照上述要点进行了重构。*

var allWatches = document.getElementsByClassName('stopwatch'), 
    firstWatch = document.getElementById('sw1')
;

// One way to call stopwatch on individual blocks
//stopWatch('sw1'); // pass id to the function

// Another way to call stopwatch on individual blocks
//stopWatch(firstWatch); // pass the element ref to the function

// One way to call stopwatch on all blocks
for(i=0; i < allWatches.length; i++) {
  stopWatch(allWatches[i]);
}

function stopWatch(element) {
  /* Declaring the html objects that will contain the time */
  var hrs, mins, secs;
  /* Declaring the html buttons */
  var start, stop, clear;
  /* Other variables */
  var seconds = 0, minutes = 0, hours = 0, t;
  // check if id is passed or the actual element reference
  if (typeof element === 'string') {
    element = document.getElementById(element);
  }
  // get reference to component elements inside
  hrs = element.querySelector('.hours');
  mins = element.querySelector('.minutes');
  secs = element.querySelector('.seconds');
  start = element.querySelector('.start');
  stop = element.querySelector('.stop');
  clear = element.querySelector('.clear');

  function runWatch() {
    seconds++;
    if (seconds >= 60) {
      seconds = 0; minutes++;
      if (minutes >= 60) {
        minutes = 0; hours++;
      }
    }

    /* Hours */
    if (hours == null) {
      hrs.textContent = "00";
    } else {
      hrs.textContent = hours > 9 ? hours : "0" + hours;
    }

    /* Minutes */
    if (minutes == null) {
      mins.textContent = "00";
    } else { 
      mins.textContent  = minutes > 9 ? minutes : "0" + minutes;
    }

    /* Seconds */
    if (seconds == null) {
      secs.textContent = "00";
    } else { 
      secs.textContent  = seconds > 9 ? seconds : "0" + seconds;
    }

    t = setTimeout(runWatch, 1000);

  } // End of runWatch
	
  runWatch();
	
  /* Start button */
  start.onclick = runWatch;

  /* Stop button */
  stop.onclick = function() { clearTimeout(t); }

  /* Clear button */
  clear.onclick = function() {
    hrs.textContentL = "00"; mins.textContent = "00"; secs.textContent = "00";
    clearTimeout(t);
    seconds = 0; minutes = 0; hours = 0;
  }	
}

* { box-sizing: border-box; margin: 0; padding: 0; }
div.stopwatch {
  width: 180px; border: 1px solid #999;
  margin: 8px; display: inline-block;
}
div.stopwatch > div { display: flex; margin: 8px; } 
div.stopwatch > div > * { flex: 1 1 auto; }
div.stopwatch > div > span { 
  text-align: center; 
  font-family: sans-serif; font-size: 1.7em;
}
div.stopwatch > div > button { 
  padding: 2px 4px;
}

<div id="sw1" class="stopwatch">
  <div>
    <span class="hours">00</span>
    <span class="minutes">00</span>
    <span class="seconds">00</span>
  </div>
  <div>
    <button class="start">start</button>
    <button class="stop">stop</button>
    <button class="clear">clear</button>	
  </div>
</div>

<div id="sw2" class="stopwatch">
  <div>
    <span class="hours">00</span>
    <span class="minutes">00</span>
    <span class="seconds">00</span>
  </div>
  <div>
    <button class="start">start</button>
    <button class="stop">stop</button>
    <button class="clear">clear</button>	
  </div>
</div>

如果你想让秒表的多个示例工作，你显然需要更多的开始/停止按钮。你可以用一个参数来调用添加时间的函数，这个参数是按钮id。按钮id会告诉函数要添加时间的秒表。当然，秒表也需要用唯一的id来定义，这样你就可以指向它：）
祝你学习顺利！