Here is how you can enabled the CSS3 features:
- Right-click on a CSS file and call the Properties dialog.
- Open the Web Content Settings page.
- There you will find a field CSS Profile set to "none".
- Change the CSS Profile value to "CSS3: Cascading Style Sheets, level 3".
- Click the OK button.
Now, try again the content assist in the CSS editor. It will suggest also the CSS3 properties like "border-radius".
This is how to change the CSS profile per file. It is also possible to change it per project using the same steps on the project's Properties page. But it only works if the project is a Dynamic Web Project or a Static Web Project one. The Web Content Settings page is not available for other project types like PHP projects. It is also not available as a workspace-wide preference. So, for the majority of project types it is only possible to change the CSS profile per file.
The good news is that with Neon M6 the default CSS profile becomes CSS3. See bug 438283. So, the above steps won't be necessary in the near future and users will see the CSS3 features off the shelf.