To create a CSS triangle, you need to use transparent border techniques: 1. Set the width to 0 and generate a triangle through the border. If border-bottom is solid, the triangle will move upward; 2. Adjust the border width to control the size of the triangle, and the color is determined by the solid border; 3. It is often used in tooltip arrows, and positioned with position:absolute; 4. The clip-path can be implemented, but the border method is simpler and more compatible. Although this method is "hack", it is reliable and effective.
Creating a triangle with CSS is a common technique used in web design, especially for toolsets, 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 neighbor 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
transparentso 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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