Why is 019 not a JavaScript syntax error? Or why is 019 > 020

From what I could find, it seems that some implementations of JavaScript just don’t follow the spec on that point.

From the MDN site:

Note that decimal literals can start with a zero (0) followed by
another decimal digit, but If the next digit after the leading 0 is
smaller than 8, the number gets parsed as an octal number. This won’t
throw in JavaScript, see bug 957513. See also the page about
parseInt().

This still doesn’t explain why 019 == 19, given that the next digit after the leading 0 is 1 and the whole number should therefore be parsed as octal. But the referenced bug does seem related to your case. Its description says:

The following JavaScript program should throw an error:

08

As per the spec, DecimalIntegerLiteral can never be 0 directly
followed by another decimal digit, although Chrome/Opera, PrestOpera,
and Firefox do support it.

The bug is closed as WONTFIX

However, 019 would be a valid decimal literal, with value equal to 19, according to the draft of the next edition:

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-additional-syntax-numeric-literals

(I’ve marked the relevant rules)

The syntax and semantics of 11.8.3 is extended as follows except that 
this extension is not allowed for strict mode code:

[...]

DecimalIntegerLiteral ::
    0
    NonZeroDigit DecimalDigits_opt
    NonOctalDecimalIntegerLiteral                         // (1)

NonOctalDecimalIntegerLiteral ::
    0 NonOctalDigit
    LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit    // (2)
    NonOctalDecimalIntegerLiteral DecimalDigit

LegacyOctalLikeDecimalIntegerLiteral ::
    0 OctalDigit                                          // (3)
    LegacyOctalLikeDecimalIntegerLiteral OctalDigit

So 01 is a LegacyOctalLikeDecimalIntegerLiteral (3) . Then 019 is a NonOctalDecimalIntegerLiteral (2) which in turn is a DecimalIntegerLiteral (1).