There is an old PR (#2112) that addressed alt text for icons. However in order to pass accessibility tests the same needs to happen with maker shadows.

I'm wondering if it's ok just set a default alt text for icons.

How to reproduce

• Leaflet version: 1.0.2
• Browser: Chrome 55
• OS/Platform (with version) I'm using: MacOS Sierra
• Create a map with default markers
• Run wave accessibility tool or similar (https://chrome.google.com/webstore/detail/wave-evaluation-tool/jbbplnpkjmmeebjpijfedlgcdilocofh)
• A list of errors related with missing alt text in makers are displayed

Acceptance criteria

Like marker icons, marker shadows should have alternative text