inconsistent browser specific behavior

[wellrus]

caused by https://github.com/vuejs/core/issues/6158

repro url

repro:

last chrome:

1. open url
2. open file Comp.vue
3. remove attribute "scoped" from style tag. both "p" tag has yellow background
4. add attribute "scoped" to style tag. only first "p" tag has yellow background

last firefox:

1. open url
2. open file Comp.vue
3. remove attribute "scoped" from style tag. both "p" tag has yellow background
4. add attribute "scoped" to style tag. both "p" tag has yellow background

expected: because there is a bug https://github.com/vuejs/core/issues/6158 firefox should have only first "p" tag with yellow background