ActiveTab

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ActiveTab makes it easier to spot the active tab in Safari on Mac by drawing a line below it. It works by predicting the position of the active tab based on the size of your browser window and the number of tabs.

"The extension [...] undeniably makes it easier to spot the active tab in Safari." - MacStories
"It's worth every penny." - iMore
"ActiveTab simply makes it easier to spot the active tab in Safari." - MacRumors
"It makes the tab instantly recognizable, even when you’re not looking directly at it" - Cult of Mac
"It solves the problem of creating enough visual contrast between active and inactive tabs" - iDownloadBlog

Caveats:
• It only works with “Separate Tab Bar”.
• It works best if you hide the Favorites Bar and turn off “Show Color in Tab Bar.”
• It will not work reliably if you have so many tabs in a window that the tab bar becomes scrollable.
• It also won’t work if you change the zoom level of web pages.

The "can read and alter webpages" text in the permission section is just unfortunate wording. ActiveTab needs access to the web page to draw the line below tabs. It's not doing anything creepy!