保证金块开始和保证金顶部有什么区别?

Question

试图理解两者之间用法的核心差异，但我找不到详细介绍这种比较的文章或文档。以此处提供的示例为例，假设以下内容：

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>

此实例与使用margin-top的实例之间的差异非常小(无论多么明显(。

规范指出，margin-block-start取决于布局模型，而margin-top是指包含块的宽度。如果有人能用外行的术语解释它，会很高兴。

Answer 1

从官方的¹规范中，您可以阅读：

这些属性对应于上边距、下边距、左边距和右边距属性。映射取决于元素的写入模式、方向和文本方向。

默认情况下，您将具有以下映射：

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

例：

.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，如果我们更改写入模式，您将看到我们将具有不同的映射。

.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，您会注意到使用vertical-lr映射等于

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

以及使用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。

这是一个更好的：

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 属性定义逻辑块启动 元素的边距，映射到物理边距，具体取决于 元素的写入模式、方向性和文本方向。

请运行代码段以查看实际行为：

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>