margin-block-start 和 margin-top 有什么区别？

Question

试图了解两者在用法上的核心区别，但我找不到详细说明此类比较的文章或文档。以here提供的例子为例，假设如下：

div {
  background-color: yellow;
  width: 120px;
  height: 120px;
}

.exampleText {
  writing-mode: vertical-lr;
  margin-block-start: 20px;
  background-color: #c8c800;
}

<div>
  <p class="exampleText">Example text</p>
</div>

this instance 和margin-top is used 之间的区别非常小（但可见）。

specifications 表示margin-block-start 取决于布局模型，而margin-top 指的是包含块的宽度。如果有人能用外行的话来解释它会很高兴。

Answer 1

来自官方¹specification你可以阅读：

这些属性对应于 margin-top、margin-bottom、margin-left 和 margin-right 属性。映射取决于元素的书写模式、方向和文本方向。

默认情况下，您将拥有以下映射：

margin-block-start = margin-top
margin-block-end = margin-bottom
margin-inline-start = margin-left
margin-inline-end = margin-right

例子：

.margin {
  margin-top:50px;
  margin-bottom:10px;
  margin-left:100px;
  margin-right:200px;
}

.margin-alt {
  margin-block-start:50px;
  margin-block-end:10px;
  margin-inline-start:100px; 
  margin-inline-end:200px; 
}

div {
  border:2px solid;
  padding:20px;
}

<div class="margin">
A
</div>
<hr>
<div class="margin-alt">
A
</div>

现在如果我们改变书写模式，你会看到我们会有不同的映射。

.margin {
  margin-top:50px;
  margin-bottom:10px;
  margin-left:100px;
  margin-right:200px;
}

.margin-alt {
  margin-block-start:50px;
  margin-block-end:10px;
  margin-inline-start:100px; 
  margin-inline-end:200px; 
}

div {
  border:2px solid;
  padding:20px;
  writing-mode: vertical-lr;
}

<div class="margin">
A
</div>
<hr>
<div class="margin-alt">
A
</div>
<div class="margin-alt" style="writing-mode: vertical-rl;">
A
</div>

在上面，你会注意到当使用vertical-lr 时，映射等于

margin-block-start = margin-left
margin-block-end = margin-right
margin-inline-start = margin-top 
margin-inline-end = margin-bottom 

当使用vertical-rl时

margin-block-start = margin-right
margin-block-end = margin-left
margin-inline-start = margin-top 
margin-inline-end = margin-bottom

---

我不会详细说明所有情况，但基本上每个margin-*-* 属性都会根据writing-mode、direction 和text-orientation 的值映射到margin-* 属性。

您可以使用不同的值来查看不同的情况。

---

您的示例有点复杂，因为您有边距折叠并且默认边距应用于p，因此很难理解。

这里有一个更好的：

div:not([class]) {
  background-color: yellow;
  width: 120px;
  height: 120px;
  border:1px solid;
}

.exampleText {
  writing-mode: vertical-lr;
  margin-block-start: 20px; /* we will end with a margin-left */
  background-color: #c8c800;
}
.exampleText2 {
  writing-mode: vertical-lr;
  margin-top: 20px; /* this will remain a margin-top */
  background-color: #c8c800;
}

<div>
  <div class="exampleText">Example text</div>
</div>

<div>
  <div class="exampleText2">Example text</div>
</div>

¹：您使用的链接是 MDN 页面，这是一个很好的参考，但不是官方规范。

Answer 2

margin-block-start CSS 属性定义逻辑块开始 元素的边距，根据元素映射到物理边距 元素的书写方式、方向性和文本方向。

请运行 sn-p 以查看实际行为：

document.addEventListener('DOMContentLoaded', event => {
const choice = document.querySelectorAll('.choice');
const target = document.querySelectorAll('.row-target')[0];

choice.forEach(el => el.addEventListener('click', event => {
  choice.forEach(el => el.classList.remove('selected'));
  event.target.classList.add('selected');
  target.setAttribute('style', event.target.innerText);
}));
})

* {
    box-sizing: border-box;
}

#container {
    width: 300px;
    height: 200px;
    display: flex;
    align-content: flex-start;
    flex-direction: column;
    justify-content: flex-start;
    margin-left: 20px;
}

.row {
    height: 33.33%;
    display: inline-block;
    border: solid #5b6dcd 10px;
    background-color: rgba(229,232,252,.6);
    flex-shrink: 0;
}

.row-target {
    border: solid 10px #ffc129;
    background-color: rgba(255,244,219,.6);
}

.transition-all {
    transition: all .3s ease-in;
}

.choice {
background-color: #fff;
    display: flex;
    align-items: center;
    flex-grow: 1;
    position: relative;
    z-index: 1;
    display: block;
    margin: .2em 0;
    width: 100%;
    border: 2px solid #d6dee0;
    border-left: 5px solid #d6dee0;
    font-size: 14px;
    cursor: pointer;
    transition: background-color .2s ease-out,border .2s ease-out;
}

.choice.selected:before {
    opacity: 1;
    right: -1.5em;
}
.choice:before {
    content: '';
    position: absolute;
    top: 50%;
    right: -10px;
    z-index: 1;
    opacity: 0;
    transition: all .2s ease-out;
    transform: translateY(-50%);
    border-left: 10px solid #1b76c4;
    border-top: 10px solid transparent;
    border-bottom: 10px solid transparent;
}

.selected {
    border-color: #1b76c4;
    border-left-color: #1b76c4;
    box-shadow: inset 0 2px 2px -2px rgb(0 0 0 / 20%);
    transition: height .5s;
    cursor: text;
}

<table>
<tr>
<td>
    <div class="choice selected">
        margin-block-start: 20px; <br>
        writing-mode: horizontal-tb;
    </div>
    
    <div class="choice">
        margin-block-start: 20px;  <br>
        writing-mode: vertical-rl;
    </div>
    
    <div class="choice">
        margin-block-start: 20%; <br>
        writing-mode: horizontal-tb;
    </div>
    
    <div class="choice">
        margin-block-start: auto; <br>
        writing-mode: vertical-lr;
    </div>
</td>
<td valign="top">
<div id="container">
            <div class="row">One</div>
            <div class="row row-target transition-all" id="example-element" style="margin-block-start: 20px; writing-mode: horizontal-tb;">Two</div>
            <div class="row">Three</div>
        </div>
        
</td></tr>
</table>