This website tests how standard and custom HTML tags are indexed by Google under different circumstances.
| Content | Javascript | Link | Seen by Google |
|---|---|---|---|
| async javascript | ES5 | /html-dynamic-async | no |
| Content | Tag definition | Shadow DOM | Javascript | Link | Seen by Google |
|---|---|---|---|---|---|
| sync javascript | immediate | no | ES5 | /noshadow-dynamic-sync-es5 | no |
| sync javascript | immediate | no | ES6 | /noshadow-dynamic-sync | no |
| sync javascript | immediate | yes | ES5 | /shadow-dynamic-sync-es5 | no |
| sync javascript | immediate | yes | ES6 | /shadow-dynamic-sync | no |
| async javascript | lazy | no | ES6 | /noshadow-dynamic-async | no |
| async javascript | lazy | yes | ES6 | /shadow-dynamic-async | no |
Google can always see static HTML content on the page, in standard or custom HTML tags.
When it comes to Javascript-rendered content, Google completely ignores custom HTML tags but not standard ones.