Why Japanese chars are so special in this case in different browsers
provided that they are all UTF-8 encoded?

They are special because they use different font and therefore are taller than Latin characters. You can see the same result if you use Latin text, but increase the font-size for that specific value span.

However the main problem is that your wrapper div elements are basically non-existent and not forcing child elements to be in rows and therefore each element is trying to be next to its sibling. If you removed the br element you would see that there is only one row.

A better solution is to use css to order your elements into rows and remove the br completely. Simple way to do that is to use flex on your wrapper elements. Also you should fix the height of rows to get a more consistent appearance (I added line-height to counter that).

.element-label {
  display: block;
  float: left;
  width: 260px;
  padding-left: .5em;
  background: #f0f0f0;
  color: #A00000;
  font-size: 9pt;
}

.element-value {
  display: block;
  float: left;
  margin-left: 1em;
  font-weight: bolder;
  white-space: nowrap;
  font-size: 9pt;
}

.row {
  display: flex;
  line-height: 9pt;
  margin-bottom: .3em;
}

<div class="row" id="field1">
  <span class="element-label">Label 1 </span>
  <span class="element-value">テスト </span>
</div>
<div class="row" id="field2">
  <span class="element-label">Label 2 </span>
  <span class="element-value">testing value</span>
</div>

.element-label {
  display: block;
  float: left;
  width: 260px;
  padding-left: .5em;
  background: #f0f0f0;
  color: #A00000;
  font-size: 9pt;
}

.element-value {
  display: block;
  float: left;
  margin-left: 1em;
  font-weight: bolder;
  white-space: nowrap;
  font-size: 9pt;
}

<div id="field1">
  <span class="element-label">Label 1 </span>
  <span class="element-value">テスト </span>
</div>
<br />
<div id="field2">
  <span class="element-label">Label 2 </span>
  <span class="element-value">testing value</span>
</div>

In MS IE:

However, in Chrome or MS Edge:
the html looks like:

The layout in IE is what I expect/want.

However, when I change to the Japanese chars to other language chars (English, Chinese, Korean, etc.).
They all look right.

1. How can I format the Japanese chars correctly in this case

2. Why Japanese chars are so special in this case in different browsers provided that they are all UTF-8 encoded?