Check doctype, character encoding and the html lang attribute.
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Three small declarations that cause outsized problems when missing: the doctype, the character set and the page language. Between them they decide whether your apostrophes render, whether screen readers pronounce your content correctly, and whether search engines know what language you are writing in.
Indirectly. It helps search engines confirm the language and it is a WCAG requirement, because screen readers use it to pick pronunciation rules.
Browsers guess, and usually guess right, but accented characters and curly quotes can render as garbage. Always declare UTF-8.
Without it browsers use quirks mode, an emulation of 1990s rendering behaviour that breaks modern CSS in subtle ways.
Either works. en-GB is more precise and worth using when spelling and terminology are British.
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