Problem
Imagine an HTML attribute is given incorrect case. Although in a web browser the attribute will be case-normalized and matched to the correct string, in yew this can produce not only an error but an unhelpful error which leads away from a solution.

Steps To Reproduce

Check out https://codeberg.org/mcc/rs-bug/src/branch/why-yew-no-compile

Run (cd client && trunk build --release)

This renders a <form> with an onSubmit= attribute with a Callback. This is incorrect (it's "onsubmit", no caps). But I didn't know that¹.

Expected behavior

Either of:

1. The HTML standard says² "all attribute names on HTML elements in HTML documents get ASCII-lowercased automatically". All HTML attributes (including data- attributes; see link) are name-lowercased by definition and the browser normalizes to lowercase when interpreting documents. Therefore, Yew should do what the browser³ does and normalize attribute names to lowercase. "onSubmit=" in a Yew html! should be treated as "onsubmit=".
2. Because it's not possible for there to be an attribute which is the same as a defined attribute but with different case, Yew should detect onSubmit case-collides with onsubmit and issue a specific warning "you have misspelled onsubmit=".

Observed behavior

error[E0277]: the trait bound `Callback<Event>: IntoPropValue<...>` is not satisfied
  --> client/src/main.rs:31:29
   |
31 |                   onSubmit={on_submit}
   |                             ^^^^^^^^^ unsatisfied trait bound
   |
   = help: the trait `IntoPropValue<Option<implicit_clone::unsync::string::IString>>` is not implemented for `yew::Callback<Event>`
   = help: the following other types implement trait `IntoPropValue<T>`:
             `&'static [(K, V)]` implements `IntoPropValue<implicit_clone::unsync::map::IMap<K, V>>`
             `&'static [T]` implements `IntoPropValue<implicit_clone::unsync::array::IArray<T>>`
             `&'static str` implements `IntoPropValue<Classes>`
             `&'static str` implements `IntoPropValue<Option<String>>`
             `&'static str` implements `IntoPropValue<Option<implicit_clone::unsync::string::IString>>`
             `&'static str` implements `IntoPropValue<String>`
             `&'static str` implements `IntoPropValue<implicit_clone::unsync::string::IString>`
             `&ChildrenRenderer<VNode>` implements `IntoPropValue<VNode>`
           and 62 others
   = note: the full name for the type has been written to '/home/mcc/work/r/ynot/target/wasm32-unknown-unknown/release/deps/fedichat_client-9c7418ab9ecea3a3.long-type-3825124899325781131.txt'
   = note: consider using `--verbose` to print the full type name to the console

For more information about this error, try `rustc --explain E0277`.
error: could not compile `fedichat-client` (bin "fedichat-client") due to 1 previous error

To a new Yew user, this is very confusing. What it means is:

• Because "onSubmit" does not match "onsubmit", Yew assumes you want an unknown/custom attribute named "onSubmit".
• By default, an attribute unknown to Yew has type string, but I passed in a callback.
• "string" isn't "string" but rather a complicated trait situation, so you don't get a simple error like "string expected"

I wasn't able to figure out this error without help.

Environment:

• Yew version: 0.22.0
• Rust version: rustc 1.90.0 (1159e78c4 2025-09-14)
• Target / Build tool: trunk 0.21.14
• OS, if relevant: Debian 13

¹ Apparently I have been capitalizing JavaScript event handlers incorrectly for the last ~28 years.
² This is a "note", I haven't found where the underlying rule is documented.
³ Also React/Preact, I've tested this