--- title: Basic markup --- - [basically what we have in the Svelte docs today](https://svelte.dev/docs/basic-markup) ## Tags A lowercase tag, like `
`, denotes a regular HTML element. A capitalised tag, such as `` or ``, indicates a _component_. ```svelte
``` ## Attributes and props By default, attributes work exactly like their HTML counterparts. ```svelte
``` As in HTML, values may be unquoted. ```svelte ``` Attribute values can contain JavaScript expressions. ```svelte page {p} ``` Or they can _be_ JavaScript expressions. ```svelte ``` Boolean attributes are included on the element if their value is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy) and excluded if it's [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy). All other attributes are included unless their value is [nullish](https://developer.mozilla.org/en-US/docs/Glossary/Nullish) (`null` or `undefined`). ```svelte
This div has no title attribute
``` Quoting a singular expression does not affect how the value is parsed yet, but in Svelte 6 it will: ```svelte ``` When the attribute name and value match (`name={name}`), they can be replaced with `{name}`. ```svelte ``` By convention, values passed to components are referred to as _properties_ or _props_ rather than _attributes_, which are a feature of the DOM. As with elements, `name={name}` can be replaced with the `{name}` shorthand. ```svelte ``` _Spread attributes_ allow many attributes or properties to be passed to an element or component at once. An element or component can have multiple spread attributes, interspersed with regular ones. ```svelte ``` > The `value` attribute of an `input` element or its children `option` elements must not be set with spread attributes when using `bind:group` or `bind:checked`. Svelte needs to be able to see the element's `value` directly in the markup in these cases so that it can link it to the bound variable. > Sometimes, the attribute order matters as Svelte sets attributes sequentially in JavaScript. For example, ``, Svelte will attempt to set the value to `1` (rounding up from 0.5 as the step by default is 1), and then set the step to `0.1`. To fix this, change it to ``. > Another example is ``. Svelte will set the img `src` before making the img element `loading="lazy"`, which is probably too late. Change this to `` to make the image lazily loaded. ## Events Listening to DOM events is possible by adding attributes to the element that start with `on`. For example, to listen to the `click` event, add the `onclick` attribute to a button: ```svelte ``` Event attributes are case sensitive. `onclick` listens to the `click` event, `onClick` listens to the `Click` event, which is different. This ensures you can listen to custom events that have uppercase characters in them. Because events are just attributes, the same rules as for attributes apply: - you can use the shorthand form: `` - you can spread them: `` - component events are just (callback) properties and don't need a separate concept Timing-wise, event attributes always fire after events from bindings (e.g. `oninput` always fires after an update to `bind:value`). Under the hood, events are either listened to directly through `addEventListener`, or the event is delegated. ### Event delegation To reduce the memory footprint and increase performance, Svelte uses a technique called event delegation. This means that certain events are only listened to once at the application root, invoking a handler that then traverses the event call path and invokes listeners along the way. There are a few gotchas you need to be aware of when it comes to event delegation: - when you manually dispatch an event with the same name as one of the delegated ones, make sure to set the `{ bubbles: true }` option - when listening to events programmatically (i.e. not through `