BESEN which stands for Bero’s Ecma Script Engine is a ECMAScript Fifth Edition implementation and appears to be written entirely in Object Pascal. BESEN looks to be open source as well. It is unclear what platforms are supported but it looks like at least Windows and possibly Linux.
Lastly, I want to mention the Delphi HTML Component Library which is a cross-patform and 100% native HTML rendering library for Delphi that brings all the power of HTML/CSS into desktop and mobile Delphi applications. The platforms it supports are Win32/64, OSX, Android, iOS and Linux (through FMXLinux).