Styling code-input elements with CSS

Contributors: 2025 Oliver Geer

code-input elements can be styled like textarea elements in most cases; however, there are some exceptions:

• The CSS variable --padding should be used rather than the property padding (e.g. <code-input style="--padding: 10px;">...), or --padding-left, --padding-right, --padding-top and --padding-bottom instead of the CSS properties of the same names. For technical reasons, the value must have a unit (i.e. 0px, not 0). To avoid overcomplicating the code, this padding is always included in any width/heights of code-input elements, so if you want to style textareas and code-input elements with best consistency set box-sizing: border-box on them.
• Background colours set on code-input elements will not work with highlighters that set background colours themselves - use (code-input's selector) pre[class*="language-"] for Prism.js or .hljs for highlight.js to target the highlighted element with higher specificity than the highlighter’s theme. You may also set the background-color of the code-input element for its appearance when its template is unregistered / there is no JavaScript.
• The caret and placeholder colour by default follow and give good contrast with the highlighted theme. Setting a CSS rule for color and/or caret-color properties on the code-input element will override this behaviour.
• For technical reasons, code-input:focus won’t match anything. Use code-input:has(textarea:focus) instead.
• For now, elements on top of code-input elements should have a CSS z-index at least 3 greater than the code-input element.

Please do not use className in JavaScript referring to code-input elements, because the code-input library needs to add its own classes to code-input elements for easier progressive enhancement. You can, however, use classList and style as much as you want - it will make your code cleaner anyway.

Methods of resizing

code-input elements default to having a fixed height and filling the width of their container while the code inside scrolls, and you can set the ordinary CSS properties (for example height and width) to change this size. You can also make the size more flexible using CSS:

• The CSS resize property (see link) can be used on code-input elements to give them the manual resizing handle textareas often come with, when the web browser supports it. For example, <code-input style="resize: both;"… gives:

• You can also make a code-input element resize automatically to fit its contents - use the Autogrow plugin for that.