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# CSS Style Objects

Many MathJax components allow you to specify CSS styles that control the look of the elements they create. These are described using CSS style objects, which are JavaScript objects that represent standard CSS declarations. The main CSS style object is a collection of name:value pairs where the name is the CSS selector that is being defined, and the value is an object that gives the style for that selector. Most often, the selector will need to be enclosed in quotation marks, as it will contain special characters, so you would need to use `"#myID"` rather than just `#myID` and `"ul li"` rather than just `ul li`.

The value used to define the CSS style can either be a string containing the CSS definition, or a javascript object that is itself a collection of name:value pairs, where the name is the attribute being defined and value is the value that attribute should be given. Note that, since this is a JavaScript object, the pairs are separated by commas (not semi-colons) and the values are enclosed in quotation marks. If the name contains dashes, it should be enclosed in quotation marks as well.

For example, `jax/output/HTML-CSS/config.js` includes the following declaration:

    styles: {

      ".MathJax_Display": {
        "text-align": "center",
        margin:       "1em 0em"
      },

      ".MathJax .merror": {
        "background-color": "#FFFF88",
        color:   "#CC0000",
        border:  "1px solid #CC0000",
        padding: "1px 3px",
        "font-style": "normal",
        "font-size":  "90%"
      }

    }

This defines two CSS styles, one for the selector `.MathJax_Display`, which specifies its text alignment and margin settings, and a second for `.MathJax .merror`, which specifies a background color, foreground color, border, and so on.

You can add as many such definitions to a `styles` object as you wish. Note, however, that since this is a JavaScript object, the selectors must be unique (e.g., you can’t use two definitions for `"img"`, for example, as only the last one would be saved). If you need to use more than one entry for a single selector, you can add comments like `/* 1 */` and `/* 2 */` to the selector to make them unique.

It is possible to include selectors like `"@media print"`, in which case the value is a CSS style object. For example:

    styles: {
      "@media print": {
        ".MathJax .merror": {
           "background-color": "white",
           border: 0
        }
      }
    }

The various extensions and output processors include more examples of CSS style objects, so see the code for those files for additional samples. In particular, the `extensions/MathMenu.js`, `extensions/MathZoom.js`, `extensions/FontWarnsing.js`, and `jax/output/HTML-CSS/jax.js` files include such definitions.

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