要創(chuàng)建CSS三角形，需使用透明邊框技巧：1. 設(shè)置寬高為0，通過邊框生成三角形，如border-bottom為實(shí)色則三角形向上；2. 調(diào)整邊框?qū)挾瓤煽刂迫切未笮?，顏色由?shí)色邊框決定；3. 常用於工具提示箭頭，配合position:absolute定位；4. 可選clip-path實(shí)現(xiàn)，但邊框法更簡單兼容，該方法雖為“hack”但可靠有效。

Creating a triangle with CSS is a common technique used in web design, especially for tooltips, dropdown arrows, or decorative elements. Since HTML elements are rectangular by default, we use a clever trick with borders to create triangles.

Here's how to do it:

1. Use Transparent Borders to Form a Triangle

The key idea is to create a box with zero width and height, then use thick borders. By setting only one border to be colored and the others transparent, you can form a triangle.

Example: A downward-pointing triangle

 .triangle {
  width: 0;
  height: 0;
  border-left: 10px solid transparent;
  border-right: 10px solid transparent;
  border-top: 10px solid black;
  border-bottom: 10px solid transparent;
}

 <div class="triangle"></div>

This creates a small upward-pointing triangle. Wait — actually, it points down ? No! Let's clarify:

• border-top: 10px solid black → the top border becomes the visible part.
• The left and right borders are transparent and help shape the triangle.
• So this creates a downward-pointing triangle .

Wait — actually, no: if only the top border is colored, the triangle points down ? That seems backwards.

Let's fix the confusion:

? The colored border points away from the direction of the triangle.

So:

• border-top: solid black → triangle points down
• border-bottom: solid black → triangle points up
• border-left: solid black → triangle points right
• border-right: solid black → triangle points left

? Corrected example: Upward-pointing triangle

 .triangle-up {
  width: 0;
  height: 0;
  border-left: 10px solid transparent;
  border-right: 10px solid transparent;
  border-bottom: 10px solid red; /* Points up */
}

2. Adjust Size and Color

The size of the triangle is controlled by the border width. Larger border = larger triangle.

 .big-blue-triangle {
  width: 0;
  height: 0;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
  border-bottom: 20px solid blue;
}

3. Common Use Case: Tooltip Arrow

You can position the triangle using position: absolute to create a small arrow beneath a tooltip.

 <div class="tooltip">
  This is a tooltip
  <div class="arrow"></div>
</div>

 .tooltip {
  position: relative;
  background: #333;
  color: white;
  padding: 10px;
  width: 150px;
  text-align: center;
}

.arrow {
  position: absolute;
  top: 100%; /* Position below the tooltip */
  left: 50%;
  transform: translateX(-50%);
  width: 0;
  height: 0;
  border: 10px solid transparent;
  border-top: 10px solid #333; /* Arrow color matches tooltip */
}

This creates a neat little arrow pointing down from the tooltip.

4. Optional: Use CSS clip-path (Alternative Method)

You can also use clip-path for more control, though it's less commonly used for simple triangles.

 .triangle-clip {
  width: 20px;
  height: 20px;
  background: green;
  clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
}

This creates a triangle pointing upward using clipping.

But for most cases, the border method is simpler and more widely supported.

Quick Tips:

• Always set the other borders to transparent so only one side shows.
• Use width: 0; height: 0; to collapse the element.
• Remember: the colored border determines the direction the triangle points .

Basically, it's a hack — but a well-established, reliable one.

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