<canvas>: The Graphics Canvas element - HTML: HyperText Markup Language 编辑

Use the HTML <canvas> element with either the canvas scripting API or the WebGL API to draw graphics and animations.

Attributes

This element's attributes include the global attributes.

height

The height of the coordinate space in CSS pixels. Defaults to 150.

moz-opaque

Lets the canvas know whether or not translucency will be a factor. If the canvas knows there's no translucency, painting performance can be optimized. This is only supported by Mozilla-based browsers; use the standardized canvas.getContext('2d', { alpha: false }) instead.

width

The width of the coordinate space in CSS pixels. Defaults to 300.

Usage notes

Alternative content

You may (and should) provide alternate content inside the <canvas> block. That content will be rendered both on older browsers that don't support canvas and in browsers with JavaScript disabled. Providing a useful fallback text or sub DOM helps to make the canvas more accessible.

Required </canvas> tag

Unlike the <img> element, the <canvas> element requires the closing tag (</canvas>).

Sizing the canvas using CSS versus HTML

The displayed size of the canvas can be changed using CSS, but if you do this the image is scaled during rendering to fit the styled size, which can make the final graphics rendering end up being distorted.

It is better to specify your canvas dimensions by setting the width and height attributes directly on the <canvas> elements, either directly in the HTML or by using JavaScript.

Maximum canvas size

The maximum size of a <canvas> element is very large, but the exact size depends on the browser. The following is some data we've collected from various tests and other sources (e.g. Stack Overflow):

Note: Exceeding the maximum dimensions or area renders the canvas unusable — drawing commands will not work.

Examples

HTML

This code snippet adds a canvas element to your HTML document. A fallback text is provided if a browser is unable to render the canvas, or if can't read a canvas.

<canvas width="300" height="300">
  An alternative text describing what your canvas displays.
</canvas>

JavaScript

Then in the JavaScript code, call HTMLCanvasElement.getContext() to get a drawing context and start drawing onto the canvas:

const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
ctx.fillStyle = 'green';
ctx.fillRect(10, 10, 100, 100);

Result

Accessibility concerns

Alternative content

The <canvas> element on its own is just a bitmap and does not provide information about any drawn objects. Canvas content is not exposed to accessibility tools as semantic HTML is. In general, you should avoid using canvas in an accessible website or app. The following guides can help to make it more accessible.

• MDN Hit regions and accessability
• Canvas accessibility use cases
• Canvas element accessibility issues
• HTML5 Canvas Accessibility in Firefox 13 – by Steve Faulkner
• Best practices for interactive canvas elements

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• MDN canvas portal
• Canvas tutorial
• Canvas cheat sheet
• Canvas-related demos
• Canvas introduction by Apple