Margin-top не работает с <label>

Ответ 1

Добавьте display:inline-block к своему правилу brtop.

.brtop {
    margin-top:5px;
    display:inline-block;
}

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div,
span,
object,
iframe,
h1,
h2,
h3,
h4,
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p,
blockquote,
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small,
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sub,
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  font-size: 100%;
  vertical-align: baseline;
  background: transparent
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body {
  line-height: 1
}

article,
aside,
details,
figcaption,
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header,
hgroup,
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nav,
section {
  display: block
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  list-style: none
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blockquote,
q {
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}

blockquote:before,
blockquote:after,
q:before,
q:after {
  content: none
}

a {
  margin: 0;
  padding: 0;
  font-size: 100%;
  vertical-align: baseline;
  background: transparent
}

ins {
  background-color: #ff9;
  color: #000;
  text-decoration: none
}

mark {
  background-color: #ff9;
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}

del {
  text-decoration: line-through
}

abbr[title],
dfn[title] {
  border-bottom: 1px dotted;
  cursor: help
}

table {
  border-collapse: collapse;
  border-spacing: 0
}

hr {
  display: block;
  height: 1px;
  border: 0;
  border-top: 1px solid #ccc;
  margin: 1em 0;
  padding: 0
}

input,
select {
  vertical-align: middle
}


/*  ^^CSS_RESET^^ --- vv CSSforsite vv */

form {
  width: 500px;
  padding: 1.5em;
  overflow: hidden;
  border: 1px solid red;
}

fieldset {
  float: left;
  border: none;
  margin-left: 1em;
}

.brtop {
  margin-top: 5px;
  display: inline-block;
}

input {}

<form name="messageform">
  <fieldset>
    <label for="username">Name:</label>
    <br/>
    <input type="text" name="username" />
    <br/>
    <label for="email" class="brtop">Email:</label>
    <br/>
    <input type="text" name="email" />
  </fieldset>
  <fieldset>
    <label>Message:</label>
    <br/>
    <textarea name="message" cols="20" rows="4"></textarea>
  </fieldset>
</form>

Ответ 2

<label> display: inline. Если вы добавите display: inline-block в .brtop, он будет уважать маржу.