[css] highlighting not working for some transition properties

As per title - CSS syntax highlighting is not working for some [animatable properties](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_animated_properties) when using CSS `transition:`, such as `background`, `background-color`, `border-bottom` and probably many more. <!-- Please search existing issues to avoid creating duplicates. --> <!-- Also please test using the latest insiders build to make sure your issue has not already been fixed: https://code.visualstudio.com/insiders/ --> <!-- Use Help > Report Issue to prefill these. --> - VSCode Version: 1.33.0 & 1.34.0 - OS Version: macOS 10.14.4 Steps to Reproduce: 1. Use HTML file with linked external CSS file. 2. Use the CSS `transition:` property to target some [animatable properties](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_animated_properties) Example (lack of) syntax highlighting...   <!-- Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes This issue also seems to exist in other code editors, such as Atom and Brackets, but I've been advised to open an issue here nonetheless. *edit* *The issue DOES NOT occur in sublime text.