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For most people, leave built-in Site Isolation enabled. It separates sites into different browser renderer processes to limit some forms of cross-site data exposure. Disabling JavaScript does something else: it stops page scripts from running, which can reduce some script-driven activity but can also break site features. The controls can be used together; script blocking is not a replacement for Site Isolation, the browser sandbox, or browser updates.
What Site Isolation and JavaScript blocking actually do
Site Isolation creates a process boundary
A browser’s Same-Origin Policy is meant to stop one site from reading another site’s data. Site Isolation adds a separate layer by placing pages from different sites in different renderer processes. Chrome also uses Cross-Origin Read Blocking (CORB) to keep certain cross-origin HTML, XML, and JSON data from being delivered to a renderer that should not access it. These mechanisms can limit the impact of some compromised-renderer attacks, enforcement bugs, and side-channel attacks such as Spectre; they do not make the browser invulnerable or replace its sandbox. Chromium’s Site Isolation overview and Chrome’s developer explanation describe the feature and its limits.
JavaScript blocking changes what pages can execute
Blocking JavaScript prevents ordinary client-side scripts from running on affected pages. That can stop some script-based tracking and behaviors, but it does not create a process boundary or block every kind of web content or attack. Sites may rely on scripts for menus, forms, sign-in flows, and core application features. Site Isolation and script blocking address different risks and can coexist.
Security: neither control is a complete shield
| Control | Primary security effect | What it does not establish |
|---|---|---|
| Site Isolation | Separates sites across renderer processes and limits access to some cross-origin data, helping contain certain renderer compromises. | It does not eliminate browser vulnerabilities, guarantee total containment, or replace the browser sandbox. |
| JavaScript blocking | Stops page scripts from executing, preventing some script-driven behaviors and tracking from running. | It does not establish site-level process isolation or make browsing safe from all malware, phishing, exploits, or tracking. |
For general browsing, Site Isolation is best treated as one layer in browser security, not as a setting to trade away for another. Turning off JavaScript may be useful for a narrower goal, but it leaves other web content and browser attack paths in place.
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Compatibility: blocking every script often breaks sites
A 2023 study by Abdul Haddi Amjad, Zubair Shafiq, and Muhammad Ali Gulzar evaluated 100,000 websites. It found that blocking all scripts significantly degraded functionality on approximately two-thirds of the tested sites. The study also classified approximately 15% of scripts as “mixed”: they combined tracking with legitimate functionality, so blocking them could break part of a site. These are results from that study, not a prediction for every browser, extension, or site. Read the study, “Blocking JavaScript without Breaking the Web: An Empirical Investigation.”
The same study found that blocking a subset of JavaScript methods reduced major breakage by 3.8× while providing the same tracking-prevention level in its evaluation. That result supports the general idea that selective controls can be more usable than an all-scripts ban; it does not guarantee that a particular blocking tool will achieve the same result on your sites.
Rank #2
Performance: isolation has resource costs; script blocking has functional costs
Site Isolation’s costs vary with platform and workload
Separating sites into renderer processes can use additional memory and may add latency during cross-site navigation. Chromium also notes that some frames can render faster when they operate in parallel. The published memory figures below are historical, tied to specific Chrome versions and conditions, and should not be read as current expected costs:
| Historical configuration | Reported memory overhead |
|---|---|
| Chrome 67 desktop, isolating all sites with many tabs open | About 10–13%, as reported by the Chromium Project. |
| Chrome 77 Android, isolating sites users log into | About 3–5%, as reported by the Chromium Project. |
These figures are not a direct performance comparison with JavaScript blocking. Current impact depends on browser version, device, tabs, and workload. Chromium’s security overview and design document discuss the tradeoffs.
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Blocking scripts may change page behavior or make features unavailable rather than making the browser faster in a uniform, measurable way. The evidence here establishes substantial functionality loss from blocking all scripts across the study sample; it does not establish a general speed gain for readers’ devices or a head-to-head speed ranking against Site Isolation.
What Chrome enables by default—and why platform matters
Google’s managed Chrome documentation says Site Isolation is enabled by default on desktop platforms as of Chrome 76, and for most sites users log into on Android as of Chrome 77. Chromium’s security overview says Android’s initial approach isolated logged-in sites on devices with at least 2 GB of RAM, with further expansion for some site types in Chrome 92. These are version-specific statements, not a guarantee of identical behavior in every current browser build or configuration. For current details, consult Google’s Chrome Enterprise and Education Site Isolation documentation.
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For most users, Google says no action is required. Managed Chrome environments can use the SitePerProcess and IsolateOrigins policies to require isolation or target selected origins. Administrators should use Google’s current policy documentation for deployment and compatibility details.
How to choose a script restriction without confusing it with isolation
- Leave built-in Site Isolation at its default unless you have a specific, informed reason to change a browser or organization-managed setting.
- If a site is misbehaving under script blocking, allow scripts for that site or use a narrower restriction rather than assuming an all-sites ban is harmless.
- Use selective blocking with realistic expectations. Some scripts mix useful site functions with tracking, so restrictions can still cause breakage and may not block every tracking method.
- Check which control you are changing. Firefox’s
DisableJitpolicy disables a site’s JavaScript just-in-time compilation engine; it does not disable JavaScript itself. Mozilla warns that JIT-disabled sites may slow down or break and that WebAssembly will not execute. See Mozilla’s Firefox site-policy reference.
Bottom line for everyday browsing
Keep Site Isolation enabled and treat JavaScript restrictions as a separate, targeted choice. Site Isolation helps contain certain cross-site risks at the process level; script blocking can reduce some script-driven behavior but frequently disrupts website functionality when applied everywhere. Neither setting replaces the browser’s other security layers.
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