svelte/packages/svelte/messages/compile-warnings/template.md

attribute_avoid_is

The "is" attribute is not supported cross-browser and should be avoided

attribute_global_event_reference

You are referencing globalThis.%name%. Did you forget to declare a variable with that name?

attribute_illegal_colon

Attributes should not contain ':' characters to prevent ambiguity with Svelte directives

attribute_invalid_property_name

'%wrong%' is not a valid HTML attribute. Did you mean '%right%'?

attribute_quoted

Quoted attributes on components and custom elements will be stringified in a future version of Svelte. If this isn't what you want, remove the quotes

bind_invalid_each_rest

The rest operator (...) will create a new object and binding '%name%' with the original object will not work

block_empty

Empty block

component_name_lowercase

<%name%> will be treated as an HTML element unless it begins with a capital letter

element_implicitly_closed

This element is implicitly closed by the following %tag%, which can cause an unexpected DOM structure. Add an explicit %closing% to avoid surprises.

In HTML, some elements are implicitly closed by another element. For example, you cannot nest a <p> inside another <p>:

<!-- this HTML... -->
<p><p>hello</p>

<!-- results in this DOM structure -->
<p></p>
<p>hello</p>

Similarly, a parent element's closing tag will implicitly close all child elements, even if the </ was a typo and you meant to create a new element. To avoid ambiguity, it's always a good idea to have an explicit closing tag.

element_invalid_self_closing_tag

Self-closing HTML tags for non-void elements are ambiguous — use <%name% ...></%name%> rather than <%name% ... />

In HTML, there's no such thing as a self-closing tag. While this looks like a self-contained element with some text next to it...

<div>
	<span class="icon" /> some text!
</div>

...a spec-compliant HTML parser (such as a browser) will in fact parse it like this, with the text inside the icon:

<div>
	<span class="icon"> some text! </span>
</div>

Some templating languages (including Svelte) will 'fix' HTML by turning <span /> into <span></span>. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, so Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.

To automate this, run the dedicated migration:

npx sv migrate self-closing-tags

In a future version of Svelte, self-closing tags may be upgraded from a warning to an error.

event_directive_deprecated

Using on:%name% to listen to the %name% event is deprecated. Use the event attribute on%name% instead

See the migration guide for more info.

node_invalid_placement_ssr

%message%. When rendering this component on the server, the resulting HTML will be modified by the browser (by moving, removing, or inserting elements), likely resulting in a hydration_mismatch warning

HTML restricts where certain elements can appear. In case of a violation the browser will 'repair' the HTML in a way that breaks Svelte's assumptions about the structure of your components. Some examples:

• <p>hello <div>world</div></p> will result in <p>hello </p><div>world</div><p></p> (the <div> autoclosed the <p> because <p> cannot contain block-level elements)
• <option><div>option a</div></option> will result in <option>option a</option> (the <div> is removed)
• <table><tr><td>cell</td></tr></table> will result in <table><tbody><tr><td>cell</td></tr></tbody></table> (a <tbody> is auto-inserted)

This code will work when the component is rendered on the client (which is why this is a warning rather than an error), but if you use server rendering it will cause hydration to fail.

script_context_deprecated

context="module" is deprecated, use the module attribute instead

<script ---context="module"--- +++module+++>
	let foo = 'bar';
</script>

script_unknown_attribute

Unrecognized attribute — should be one of generics, lang or module. If this exists for a preprocessor, ensure that the preprocessor removes it

slot_element_deprecated

Using <slot> to render parent content is deprecated. Use {@render ...} tags instead

See the migration guide for more info.

svelte_component_deprecated

<svelte:component> is deprecated in runes mode — components are dynamic by default

In previous versions of Svelte, the component constructor was fixed when the component was rendered. In other words, if you wanted <X> to re-render when X changed, you would either have to use <svelte:component this={X}> or put the component inside a {#key X}...{/key} block.

In Svelte 5 this is no longer true — if X changes, <X> re-renders.

In some cases <object.property> syntax can be used as a replacement; a lowercased variable with property access is recognized as a component in Svelte 5.

For complex component resolution logic, an intermediary, capitalized variable may be necessary. E.g. in places where @const can be used:

{#each items as item}
	---<svelte:component this={item.condition ? Y : Z} />---
	+++{@const Component = item.condition ? Y : Z}+++
	+++<Component />+++
{/each}

A derived value may be used in other contexts:

<script>
	// ...
	let condition = $state(false);
	+++const Component = $derived(condition ? Y : Z);+++
</script>

---<svelte:component this={condition ? Y : Z} />---
+++<Component />+++

svelte_element_invalid_this

this should be an {expression}. Using a string attribute value will cause an error in future versions of Svelte

svelte_self_deprecated

<svelte:self> is deprecated — use self-imports (e.g. import %name% from './%basename%') instead

See the note in the docs for more info.