HDR
Half-Life 2: Lost CoastHigh Dynamic Range (HDR) rendering simulates brightness above that which a computer monitor is actually capable of displaying, before downconverting to 
Standard Dynamic Range (SDR, internally referred to in Source and Source 2 as LDR, for Low Dynamic Range) video. This mainly involves "blooming" colours above 100% brightness into neighboring areas, and adjusting a virtual camera aperture to compensate for any over-exposure that results.
Beside the obvious effects of this (see right), HDR gives richer colors and finer gradients: since bright and dark areas are pushed into white and black, correctly-exposed areas are drawn with a far wider range of values.
HDR rendering is currently available on Source 2006 and later, as well as all GPUs that support DirectX 9 and newer for Windows (or OpenGL 1.4 and later for macOS/Linux).
Since 
Left 4 Dead engine branch and later games, HDR rendering is now required and LDR/SDR rendering has been deprecated, with the option to disable HDR in game settings was removed outside of console commands. Despite this, 
Left 4 Dead 2, since Aug 22, 2023 update, still receiving bug fixes related to LDR skyboxes, when running on low settings.
HDR in Source
Source does not have a physically accurate HDR simulation. Its camera has a far wider range than the human eye (let alone real cameras), and it also adjusts to changes in brightness far faster. Both of these are expedient to gameplay of course, especially in multiplayer.
In fact, by default, Source does not even create "proper" HDR images. Instead, it collapses the image down to LDR early, like many other games in the 2000s/early 2010s, this is hardware limitations at the time and proper HDR output is only later introduced in mid-2010s with supported games, graphics card, displays, operating systems (Windows 10 and later), and newer HDMI/DisplayPort versions. The average user would be hard pressed to tell the difference, however, and the benefits include support for all DX9 GPUs, MSAA compatibility, and excellent performance.
improve HDR?