It’s useful to know what you did to get the error. Anyway, quick search says it happens on package build and most likely the reason is related to loaders
Just to report that issue still happens on latest versions.
column: 1
file: "/inline"
input: {line: 1, column: 1, source: "1", file: "/inline"}
line: 1
message: "Error while parsing 'inline' CSS style sheet.\n/inline:1:1: Unknown word"
name: "CssSyntaxError"
reason: "Unknown word"
source: "1"
stack: "CssSyntaxError: Error while parsing 'inline' CSS style sheet.
/inline:1:1: Unknown word
at e.error (uxp://uxp-internal/domjs_scripts.js:2:202997)
at e.unknownWord (uxp://uxp-internal/domjs_scripts.js:2:389868)
at e.other (uxp://uxp-internal/domjs_scripts.js:2:385258)
at e.parse (uxp://uxp-internal/domjs_scripts.js:2:384213)
at t.default (uxp://uxp-internal/domjs_scripts.js:2:132747)
at new e (uxp://uxp-internal/domjs_scripts.js:2:217447)
at e.process (uxp://uxp-internal/domjs_scripts.js:2:215540)
at h (uxp://uxp-internal/domjs_scripts.js:2:196909)
at parseCSSDeclaration (uxp://uxp-internal/domjs_scripts.js:2:197284)
at g._getCssProperties (uxp://uxp-internal/devtools_scripts.js:1:30074)"