These days, I am working on Intel Sandy Bridge architecture OpenGL support.
If Intel Ivy Bridge is already pretty hard to support with its OpenGL 4.0 support, Sandy Bridge is another kind of challenge: OpenGL 3.1 support with no core profile. Unfortunately, released in 2011, Sandy Bridge is a bad GPU with 2009 OpenGL support that is a still a good CPU architecture largely used as Unity stats shows.
Sandy Bridge (HD 2000 / HD 3000) is 9.8% of the Unity editor users and one of the most used GPU architectures.
In practice, Sandy Bridge supports OpenGL 3.1 with all the OpenGL 3.2 extensions but ARB_texture_multisample.
Additionally, Sandy Bridge supports six OpenGL 3.3 extensions: ARB_vertex_type_2_10_10_10_rev, ARB_timer_query, ARB_texture_rgb10_a2ui, ARB_shader_bit_encoding, ARB_occlusion_query2 and ARB_explicit_attrib_location.
Finally, Sandy Bridge supports three OpenGL 4.0 extensions: ARB_texture_query_lod, ARB_texture_buffer_object_rgb32 and draw_buffers_blend.
By giving up on feature support, basically at the release of an architecture, Intel has been able to provide pretty good drivers on the lastest architectures (Haswell / Broadwell), both in term of quality and feature set (OpenGL 4.3).
However, it is very unfortunate that anything older is more complex to support than a good old GeForce 8, released 9 years ago. Sadly, it can get worse! Older architectures than Sandy Bridge, that is GMA (OpenGL 1.4), GMA HD (OpenGL 2.0) and Ironlake HD Graphics (OpenGL 2.1) represent 12.9% of the editor users.
Lastly, Intel GPU names are dreadful: It was aweful back at the GMA time, it is still just as bad. For example, "Intel HD Graphics" (without any numbering) has been used for Clarkdale, Arrandale, Sandy Bridge, Ivy Bridge, Haswell, Bay Trail, Broadwell and Braswell.