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Pages don't need a fallback to non-Javascript in 2019. Accessibility technologies now fully support a JS enabled web, with accessibility standards following suit. That leaves only those who have voluntarily disabled JavaScript ( I've worked on public facing government websites (not 18F). We simply don't support this edge case, and our legal department supports our legal right to do so (in particular in relation to AD…
Working without JS is still a core requirement of GDS sites; c.f. https://www.gov.uk/service-manual/technology/using-progressi... and for more as to why: https://gds.blog.gov.uk/2013/10/21/how-many-people-are-missi...
They're now building sites to meet WCAG 2.1 which does support the usage of JavaScript for accessible users.